Portable Air Compressors: Everything You Need To Know
(Camp Addict does NOT accept payment from any company to review or endorse their products.)
Making sure your tires are properly inflated is something every driver should take very seriously.
Don't be that person who is driving down the road with an overloaded vehicle and tire sidewalls bulging because they are severely under-inflated.
(Unless you really want to make the nightly news.)
In this guide, you will learn about your RV tires, tire pressures (which to choose), the difference between an air compressor and a tire inflator, and much more.
You will become 'fluent' in all things about portable air compressors.
Let's get started.
Portable Air Compressor Guide
Here you will learn about why proper air inflation is important, how to determine how much air goes into your tires, and what features to look for when shopping for a portable air compressor.
It's not all dry, boring stuff (OK, maybe it is), but it's very important.
It's definitely important information you should have a basic understanding of.
Your tires, and possibly your life, will thank you.
Importance Of A Properly Inflated Tire
So what's the big deal about tire air pressure? Why should you care?
I mean, aren't these tire thingies made of solid rubber? Oh, you mean they have actual air in them?!?!?
We're pretty sure you already knew vehicle tires have air in them. You probably DON'T know how important it is to monitor and maintain correct tire pressure.
If you do, you are way ahead of most of the population.
But why exactly should you care how much air is in your tires? In other words, why is air pressure so important?
First off, tires are designed to carry a certain load on them.
The load consists of the weight of the vehicle the tires are installed on.
The load that a tire is capable of supporting (up to the maximum load rating) is dependent on the air pressure inside the tire.
Generally, the higher the tire pressure, the greater the load a tire can support. (ALWAYS follow the recommended tire pressure as provided by the vehicle manufacturer.)
Besides the load-carrying capability of a tire, air pressure affects the following:
- Tire tread wear - over/under inflated tires don't wear evenly
- Tire sidewall life - under inflated tires cause excessive heat buildup in the sidewalls, causing them to prematurely fail
- Fuel economy - under inflated tires have higher rotational friction, which burns more gas
- Road damage - correct tire inflation ensures that the tire can stand the impact of road hazards (debris, potholes, etc)
Should I Lower Pressure To Make My Ride Smoother?
Many people will lower the tire pressure in search of a better ride for their RV. This may (and that's a big MAY) help to some extent, but you have to make sure you never reduce the tire pressure to a point where the tire's load carrying capability is below the actual load put on the tire.
It's a really bad idea to put in less tire pressure then the minimum called for by your vehicle manufacturer in the name of a better ride. Unless you have a REALLY good reason for doing so, DON'T!
And normally you won't (a good example of when you might want to do this is low speed off-roading, which you aren't gonna do in your RV).
How Often Should You Check Tire Pressure?
The general rule of thumb is to check your tire pressure at least once a month. This is the bare minimum!
It is MUCH smarter to check tire pressure before the start of every trip (always check tire pressure when the tires are cold).
It wouldn't hurt to check tire pressure each day of a road trip.
All new passenger vehicles sold in the United States as of 2008 are required to have a tire pressure monitoring system (TPMS) that alerts the driver when a low tire pressure situation is present.
While this is a great safety device, it isn't a replacement for physically checking your tire pressure. TPMS systems will not alert you if there is an over inflation situation or if a tire is slightly under inflated.
They will only alert you once a tire is severely under inflated.
RVs generally do not come equipped with tire pressure monitoring systems (good luck finding a travel trailer that comes equipped with a TPMS from the factory).
There are aftermarket systems you can install (and you should!), but again, even if you have a TPMS system installed on your RV, you should regularly physically check tire pressure on ALL of your tires.
Tire Pressure Gauge
We've established why you should keep tabs on your vehicle's tire pressure on a regular basis, but does this mean you need to lug out your portable air compressor in order to use its built-in pressure gauge just to check tire pressure? Um, no. What a pain that would be!
Instead, you should get an inexpensive, yet accurate, tire pressure gauge like the Accutire MS-4021B Digital Tire Pressure Gauge shown below.
This particular tire pressure gauge is the one that Camp Addict Co-Founder Marshall has been using for longer than he can remember.
According to Marshall, 'do yourself a favor' and pick up the air pressure gauge below.
It is very inexpensive, easy to use, easy to read, and should be in every vehicle's glove box.
Portable Air Compressor Terminology
There are a lot of options when it comes to small air compressors, but not all are created equal.
During your quest to find the best portable air compressor for car tires you need to know what you are looking for and what features you should be comparing.
Below are some of the key air compressor terms you should be aware of.
Duty Cycle (Run Time)
Duty cycle is the amount of time in a one-hour period that a portable tire inflator can be run nonstop before it needs to be 'rested' to cool off.
For example, if the duty cycle is 25 minutes, the portable compressor can be run for 25 minutes out of a 60 minute period and should be 'rested' the remaining 35 minutes.
This ensures that the compressor and pump motor don't overheat (though the best portable air compressor will have thermal overload protection to prevent damage to the motor).
Duty cycle can also be represented by a percentage, so a 33% duty cycle would be 1/3 of an hour, or 20 minutes.
Therefore, a 33% duty cycle portable air compressor can be run continually for 20 minutes before needing to be 'rested' for the next 40 minutes.
Duty Cycle Considerations
A small air compressor is going to have a specified duty cycle rating which tells you how long you can continuously operate it before it needs to 'rest' (cool down).
Two things to consider:
1) The reason a duty cycle exists is heat. The compression of air creates heat. Therefore, the compressor itself has to be designed to withstand a certain amount of heat.
Air compressors cannot run continuously all day long. They will overheat. A quality portable air compressor will be able to protect itself so that you cannot use it to the point of it being damaged by heat.
Some of the Viair compressors that we review have a thermal protector that automatically resets, so if you run them too long (exceed duty cycle, or operate in a hot environment) they will protect themselves if a certain internal temperature is reached.
Once this temperature is reached, the Viair compressor will shut itself off. It will then need to rest about 30 minutes before it resets itself (once it has cooled down), at which time you can resume using it.
2) You Likely Won't Use The Compressor Non-Stop. Using a 33% duty cycle (20 minutes out of every hour) as an example, you are probably not going to run a portable air pump for 20 minutes non-stop. With a Viair compressor, it doesn't take 20 minutes to fill up a single tire.
You aren't going to be continually running the compressor as you fill up all the tires that need air. You will be 'pausing' as you check the tire pressure and move the air chuck from one tire to another.
Furthermore, most likely you are only going to be adding air to a tire that already has air in it. (Worst case scenario for most people is that you aired down your tire to go off-roading and it has 20 pounds of pressure in it but you need to add air to increase the pressure to be able to roll down the highway.)
There are 100% duty cycle small air compressors available, which means they don't have to be cooled off for a certain number of minutes each hour.
These compressors achieve a long duty cycle by having a lower output (CFM - see below) than their counterparts with a lower duty cycle.
The lower output means the compressor and motor aren't working as hard, thus not generating as much heat.
Mini air compressor duty cycle is based upon a certain outside temperature. Viair portable air compressors have a duty cycle rating based upon 72º F.
Temperatures higher than this may cause the compressors duty cycle to be lowered as the unit may take longer to cool off (and heat up quicker in the higher outside temps).
Extra Caution In Hot Places
Regardless of the duty cycle rating, a small compressor will need sufficient cooling off time between use periods.
The higher the outside air temperature (or the more severe the operating environment) will cause heat to dissipate slower and more 'rest' time will be required.
Cubic Feet per Minute (CFM)
Cubic Feet per Minute is the measure of how much air a portable air pump is able to send to the tire it's inflating.
In other words, how much air the compressor is able to send down the air hose to be used to fill up your tires.
The larger the CFM rating a portable compressor has, the quicker it will be able to inflate your tires. However, a compressor with a higher CFM rating will be larger, which requires more power from your vehicle (higher amperage draw).
This isn't necessarily a bad thing, just that a more powerful compressor (higher CFM) will need to be connected directly to your vehicle's battery as a 12-volt outlet won't be able to supply the necessary power.
A compressor will have a different CFM rating depending on the air pressure it is 'pressing' against.
For example, if a tire is completely flat (0 PSI), a compressor will have zero resistance to 'press' against and will be able to have a higher output. Therefore, a compressor has it's highest CFM rating at 0 PSI.
As the pressure inside the tire increases, the compressor has to work harder to put more air in, so its CFM rating decreases as the tire pressure increases.
For example, the Viair 400P Automatic has a 2.30 CFM rating at 0 PSI but drops down to 1.54 CFM at 60 PSI.
A small air compressor with a 100% duty cycle will have a lower Cubic Foot per Minute rating than a similarly sized compressor with a lower duty cycle (such as 33%).
This is because the compressor with the 100% duty cycle is designed to be run longer without resting so it can't be working as hard.
The more a compressor 'works' the more heat it produces, so a long duty cycle requires a cooler running compressor.
Viair Explains Cubic Feet per Minute (CFM)
Viair has a couple of automatic mini air compressor models, but what in the heck does this mean?
It simply means that the compressor can turn itself off automatically (without you having to physically turn it off) when you pause filling up a tire.
This is usually done by releasing the trigger of an inflation 'gun'. Being able to 'pause' an air compressor serves two purposes:
1) You can move from one tire to another without having to shut off the compressor. Many (including some Viair models reviewed) portable compressors require you to turn them off before you disconnect the air chuck from the tire to move it to the the next tire.
2) It allows you to get an accurate tire pressure reading. With a typical small portable air compressor, the built-in pressure gauge isn't accurate when the pump is actively putting air into the tires.
This means in order to see how much air is in the tire, you need to switch off the pump, take the pressure reading, and turn the pump back on if more air is needed.
Viair automatic air compressors let you keep the compressor turned on and still be able to get an accurate reading on the current tire pressure.
You simply release the trigger of the air inflation gun to stop air from flowing into the tire, look at the built-in gauge on the inflation gun, and then press the trigger again to continue filling up the tire if needed.
Simple! And no rushing back to the compressor to turn it off in order to read the air pressure.
This ability of the air compressor to turn itself off when you aren't 'asking' it to actively fill a tire also allows you to move from one tire to another without having to physically flip the power switch to the 'off' position, then back 'on' when you have attached the air chuck to the next tire.
Viair makes the 400P and 450P models in an automatic version.
What Pressure Should Your Tires Be Inflated To?
Since the whole purpose of a portable small air compressor for car tires is to get air into your vehicle's tires, it might be a good idea to understand how much air you should put into them.
The amount of air inside a tire is expressed in PSI (pounds per square inch), which is the unit of measure used to express how 'full' of air a vehicle tire needs to be.
A tire pressure gauge will measure the PSI of a tire.
Check Pressure When Tires Are Cold
Check your tire pressure when the tires are cold (before you have driven the vehicle for the day).
The recommended tire pressure is based upon a cold tire PSI reading. As you drive, road friction heats up the tire, which in turn heats up the air inside the tire. As the air heats up, the tire pressure increases (slightly).
Tire pressures given by vehicle manufacturers factor in this heat buildup as the tires go down the road. Checking cold tire pressure ensures the operating (hot) tire pressure is what the vehicle manufacturer intended it to be.
OK great, tire inflation pressure is measured in PSI, but that still doesn't answer the question!
What pressure should your vehicle's tires be inflated to? Well, like many things in life, it depends. (sigh)
Fortunately your vehicle manufacturer will normally tell you what PSI to inflate the tires, so it's simply a matter of finding out where this information is.
Car, trucks, and SUVs have a tire data placard inside one of the door jambs (start by looking at the driver's door).
Assuming you are still using the factory installed tire size and type, this will be the proper cold tire pressure.
Front and rear tires may have different pressures (but in the example below, they are the same). RV's will have a similar tire data placard somewhere on the outside of the rig.
Inflate To Match The Load
Certain types of tires will have a variable tire pressure depending on tire loading. This includes ST (special trailer tires - used on travel trailers) and RV tires.
The tire manufacturer will have available a tire load and inflation table for specific tire models and sizes.
You will locate the model of your tire and the size and then determine the correct inflation pressure based upon the load on that tire (axle load divided by number of tires on the axle).
Always inflate all tires on a single axle to the same pressure.
For example, Goodyear's RV tire load and inflation information can be found here (PDF). This document includes information on the Goodyear Endurance Special Trailer (ST) tire, as well as their motorhome (RV) tires.
Notice how for a specific tire size, as the tire pressure goes up, the load the individual tire can handle goes up (to the maximum rated load).
The Goodyear RV tire website states the following:
Correct tire inflation is a key component in tire care. The recommended maximum inflation pressures for your tires are indicated on the certification label or in your owner's manual. Since RVs can be loaded with many different configurations, the load on each tire will vary. For this reason, actual air pressure required should be determined based on the load on each individual tire. Inflation pressure should be adjusted to handle the tire carrying the heaviest load, and all tires on the axle should be adjusted to this standard.
How do you know what the load is on your axles/tires? You weigh your rig!
If you have a travel trailer, you can follow our instructions on weighing a travel trailer to determine the load on each axle.
Portable Air Compressor Features To Consider
- Power cord and air hose length - You need to have a power cord that's long enough to reach the power source and have the compressor sitting safely outside. And you need to have an air hose that reaches from the compressor to your tire with ease.
- Power cord and air hose quality - a cheap air hose and/or power cord will fail after being used a while. Look for a portable air pump that has an air hose that will stand up over time. You need an electrical cord that not only is heavy enough gauge wire to handle the power load, but also has quality electrical connections.
- Gauge legibility - most (all that you'd want to consider) mini air compressors come with an air pressure gauge. But you need to be able to read it or it doesn't do you any good. Make sure the gauge has graduations small enough so you can actually read the tire pressure. If you cannot tell if the gauge is reading 30 PSI or 35 PSI because there are no markings, then it's pretty useless.
- Gauge accuracy - the built in tire pressure gauge doesn't do you much good if it doesn't accurately indicate the tire pressure. You want a gauge that reads within a very small percentage of the actual pressure or spot on.
- Work light - illuminates the area so you can see to inflate your tire in the dark. This feature is debatable.
- Duty cycle (run time) - how long you can run a small compressor before it has to be rested in order to cool off.
- Output (CFM) - how much air the compressor is capable of outputting in a minute, which indicates how quickly it will inflate a tire.
- Thermal Protection - air compressors get hot when they run, and under certain circumstances they get too hot to function properly. This is why a compressor has a duty cycle. The best portable air compressors have thermal protection, which will shut down before getting too hot. The Viair 400P-RV and 450P-RV have an automatic reset thermal protector. They shut down before getting too hot, and become available when it's cooled off enough.
- Automatic compressor - the ability of a compressor to turn itself off when you release the trigger of the air inflation gun.
- Tire inflation speed - how quickly a portable air pump will inflate a tire from a totally flat condition or from a partially inflated to fully inflated condition. Tire inflation speed is a direct result of the compressor output.
- Carrying case - Gives you a place to centrally store your compressor, air hose, air fittings and other accessories. All Viair compressors reviewed here include a case.
- Overall quality (body, feet, gauge, hose, cord, etc) - there are a lot of portable tire inflators on the market, including a lot of really poorly built compressors. Unless you like to buy a new portable compressor on a semi-regularly basis, look for one that is built with quality components.
Tire Inflator vs. Portable Air Compressor
Let's just get this out there - technically ALL devices that inflate tires are 'tire inflators'. But there is a real difference in what we are talking about.
We are referring to the different styles of 12-volt devices that can be used to put air into vehicle tires.
In this case we are referring to the style of devices (inflator versus air compressor), not what they do (put air in tires, or inflate tires).
Got it? Good!
Now that we are (crystal?) clear on what we are talking about in this section, let's get to it!
The items we reviewed on this page are portable air compressors, so they fall into the 'air compressor' column. The other style of doohickey (or is it a whatchamacallit?) are tire inflators.
What's the difference??? AND why are only small air compressors reviewed on this page if a tire inflator will, well, inflate a tire?
We'll make this simple - tire inflators are fine if you have lots of time on your hands and enjoy hearing a small, not very powerful air compressor go on and on and on.
Even the best portable tire inflators have very small compressors and can take forever to fill up a deflated tire.
In other words, why would you want to use a portable tire inflator when there are MUCH better options (12-volt portable air compressors)?
What Size Are My Tires?
Portable air compressors are rated to fill tires up to a specific size (diameter).
This is because a larger diameter tire contains more air and requires a compressor with a larger air output in order to fill it up in a reasonable amount of time.
You need to match the air compressor output with the largest tire you will be inflating so that you don't spend half your day waiting around for it to finish.
There are two ways to find out how large your tire is (what diameter it is).
You will want to do this with the largest tire you will be inflating (if you have a large RV tire you want to inflate, measure that one).
This will mean your portable tire inflator will not only be capable of inflating smaller tires, but it will have the capacity to inflate the largest tire you have.
There are two methods of figuring out tire diameter.
Measure using a tape measure or enter your tire's specifications into an online calculator and let it tell you the tire diameter.
Direct measure tire diameter - Using a tape measure, go up to the largest tire you will be inflating and measure it. Easy? Yup!
But you need to measure it correctly.
Measure the diameter by holding the tape measure vertically at the center of the wheel assembly, with the end (the 'zero' point) on the ground and reading the diameter at the top of the tire.
See the picture below for a visual reference. The tire in the picture below is 30 inches in diameter.
Online calculator - In order to use an online calculator to figure out tire diameter, you will need to know the specifications of the tire itself. These specs are presented in the following format: LT265/60R18.
You can read this directly from the tire itself (choose the largest tire you will be inflating).
If your vehicle or RV has the original size tires still installed, you can also find tire size information by locating the tire data plate located on the door jamb of your car/truck or on the side of your RV.
The letters at the front of the specs (P, LT, ST, etc) indicate what type of use the tire is designed for. You can ignore this as you are only interested in the numbers.
The numbers designate certain measurements of the tire, which we won't go into here.
If you are interested in knowing exactly what these numbers mean, Goodyear has a good explanation of this.
Once you've located your tire specifications, you can 'plug' this information into this online calculator (three blank boxes in the left column titled 'Tire Calculator').
Once you've entered the correct information into the three boxes and click the 'calculate' button, the tire diameter will be displayed.
Press-On Chuck Or Twist-On Air Chuck?
First off, what in the heck is a 'chuck'? It's not that guy that was the superstar athlete in high school who is now the night manager at the local burger joint. Heh.
An air chuck is the part of a portable air pump that attaches to your vehicle's tire (specifically the tire's valve stem).
Air chucks come in two 'flavors' - a twist-on (or screw-on) style and a press-on style.
With a screw-on (twist-on) air chuck you have to physically twist (or thread) the chuck's female threads onto the valve stem's male threads.
This makes for a very secure, leak-resistant connection but is slightly slower than the press-on style to screw and unscrew from the valve stem.
A press-on air chuck utilizes a quick-connection system that fits over the top of the tire valve stem and you flip down a lever that secures the connection (by expanding the inside of the chuck).
This is definitely quicker, but may be susceptible to debris getting inside the chuck and degrading the sealing capability over time.
Here's a quick breakdown of the pro's and con's of each style of air chuck:
Screw-On Air Chuck
Press-On Air Chuck
So which type of air chuck is right for you? It depends. Don't you 'love' that answer?
If you are an off-roader that uses your air compressor in harsher environments where there is the possibility of debris (sand, dirt, etc) getting into the chuck body, go with a twist-on style chuck.
These are easier to remove debris from as you can just blow out the threads, do a quick visual check to make sure things are clear, and get back to it.
Whereas a press-on chuck's interior material is more susceptible to damage from said debris, and is harder to tell if it's free and clear inside.
However, if your vehicle(s) rarely leave pavement and you aren't using your portable air pump in the dirt, then a press-on chuck may be the way to go.
As long as you keep the inside of the chuck free of debris, it should give you years of trouble-free service.
I (Camp Addict co-founder Marshall) have been using a mini air compressor with a press-on chuck for close to 15 years (the same compressor) and it still is working great.
Then again I'm not a hard-core off-roader dragging it through the dirt and I tend to take care of my equipment.
Choose which style of air chuck you think works best for you and then use your compressor!
Don't think too hard about it. Go with your gut and get to filling your tires with air.
12-Volt Outlet Or Direct Clamp To Battery?
If you are shopping for a portable air compressor for your car or truck there is one VERY important item you need to consider before you decide.
How will you provide power to your small compressor?
The name '12-volt air compressor' implies that a 12-volt power source is required. Your vehicle has a 12-volt electrical system and thus can provide the needed 12-volts of power.
But it isn't that simple.
There are two ways to get the needed power to your portable compressor:
- 12-volt (cigarette) outlet
- By directly clamping the power leads to your battery.
This is the round-style electrical plug that you can use to power various low power consuming appliances.
The Viair compressors we review that use a round power connector that plugs into a 12-volt outlet require an outlet rated for at least 15 amps or 180 watts.
This is a large amount of power, and many 12-volt outlets won't be able to provide this much power.
How do you know how many amps your 12-volt power outlet is rated for? The power rating will either be noted at the outlet itself (on the outlet door or above/below the outlet opening) or it will be indicated in the owner's manual.
What happens if you try to plug in a portable air compressor into an outlet that isn't capable of handling the compressor's electrical load?
Best case scenario is that the fuse that protects the power outlet will blow and you will have to replace said fuse.
Worse case scenario (and this would only happen if the power outlet was wired improperly without a fuse protecting it) is that the outlet wiring would get too hot, potentially melt, or even start an electrical fire.
Yeah, it's kinda important that you don't 'ask' too much of a power outlet by plugging in something that requires more amperage than the outlet is capable of supplying.
If you are unable to determine the maximum number of amps your 12-volt outlet is capable of providing, then go with a portable air compressor with a battery clamp power cord.
Direct Clamp To Battery
If your vehicle doesn't have a 12-volt outlet capable of providing the amount of amperage that the portable air compressor for car tires requires (minimum 15 amps for Viair compressors equipped with a 12-volt 'plug' reviewed on this page), then you will need to get a compressor with power cords that directly clamp to your vehicle's battery.
This allows the small compressor to pull as many amps as it needs directly from the battery without worrying about any power outlet fuses blowing.
This style of portable air compressor has an inline fuse on the positive cable.
This protects the compressor (and wiring) from too high of amperage draw since the vehicle itself isn't providing the protection via built-in fuses.
(You are bypassing any vehicle protection by connecting directly to the vehicle battery.)
You need to be able to access your vehicle's battery in order to clamp the air compressor power cord to it, and you have to be confident in connecting/clamping to the battery.
Just make sure the compressor power switch in the 'off' position before connecting directly to the battery.
Also, in order to reduce sparking potential, it's smart to connect the negative (black) clamp to the battery last and remove it first when disconnecting.
You should now have a better understanding of portable air compressors and understand why it's important to keep an eye on air pressure.
Your tires are one of the most important parts of your vehicle, including RVs, for safety. Having them properly inflated is just one basic part of tire maintenance that you should keep up with.
If you have a portable compressor, then you are able to easily maintain proper tire pressure, no matter where you are.
Happy motoring and here's to many years of healthy tire life!
Kelly Beasley is co-founder of Camp Addict and loves sharing her enthusiasm for the RVing lifestyle. As a full-time RVer since May 2015, Kelly's playful writing style helps make learning about the sometimes dull subject of RV products a bit more interesting.
Camp Addict co-founder Marshall Wendler brings his technical expertise to help explain RV products in an easy to understand fashion. Full-time RVing since April 2014, Marshall loves sharing his knowledge of the RV lifestyle.