I think most RV owners understand the importance of maintaining proper tire inflation pressure. The problem is, if you wait until you’re on the road to check tire pressure the tires are too hot for correct evaluation. Checking the inflation pressure when you stop to refuel doesn’t make sense either; you will get higher pressure readings and if you let air out the tires they are under-inflated when they are cold again.
And, I don’t recommend using air compressors at gas stations for two reasons, one is they are abused and you don’t know if the inflation pressure is accurate, and two is if you check the tire pressure when the tires are hot you get inaccurate readings, which can be dangerous.
This is why you might want to consider purchasing a portable air compressor. But, understanding portable air compressors can be confusing, especially if you are trying to find one that is capable of increasing the inflation pressure in large motorhome tires.
Here are my top 7 tips on choosing the right portable air compressor for the job
1) Air Compressor Ratings:
Air compressors have 3 ratings you need to be concerned about: horsepower (HP), Cubic Feet Per Minute (CFM) and Pounds Per Square Inch (PSI). Understanding these 3 functions of an air compressor can help in selecting the right portable air compressor for the job.
2) Horsepower (HP):
Unfortunately, when it comes to air compressors some manufacturers don’t do a great job explaining how the horsepower rating works, (almost to the point some would consider it false advertising). Manufacturers tend to over-inflate horsepower ratings. Think about it; if the manufacturer advertises 6-horsepower rather than 2-horsepower the customer is more likely to purchase the higher rated compressor. We are programmed like this from childhood, whether it’s a lawnmower or a car, more horsepower is better. But, if you understand horsepower, as it relates to air compressors you can discover the truth.
3) Horsepower & Watts
Horsepower was invented by an engineer by the name of James Watt. The same watt you think about when buying a light bulb. Watts are used as a measure of electrical and mechanical power. When we apply watts to horsepower, 1-HP is equivalent to 746 watts. So, if you buy an air compressor rated at 6-HP and it operates on 120-volts it is, in a sense a false rating. Here’s why, if you plug the compressor into a standard 15-amp, 120-volt circuit it would only produce about 2- horsepower (120 volts X 15 amps = 1800 watts). This is slightly over a 2- horsepower rating.
The higher 6-horsepower rating advertised by the compressor manufacturer is commonly referred to as peak horsepower, similar to a vehicles engine’s horsepower rating. The peak HP rating is only accurate when the engine is running at a higher RPM range (usually 5,000 to 6,000 RPMs), but rarely do you drive your vehicle at 5,500 RPM’s. As for the air compressor, it would require more amps, like from a 240-volt circuit, to produce this peak HP rating.
4) Cubic Feet Per Minute (CFM):
Cubic feet per minute is an important rating when it comes to portable air compressors. CFM is basically a measurement of the rate at which a compressor can deliver a volume of air. CFM varies with atmospheric pressure, temperature and humidity. Air compressor manufacturers calculate Standard Cubic Feet per Minute (SCFM) as cubic feet per minute at sea level at 68 degrees F, and 36% relative humidity. Note: CFM ratings can be skewed by the manufacturer too (see below)
5) Pounds Per Square Inch (PSI):
PSI can help put things in perspective. For air compressors the CFM ratings are given at a specific PSI. In other words, if an air compressor is rated at 3 CFM @ 90 PSI it should be capable of delivering the 3 CFM at 90 PSI. The problem is, air compressor manufacturers can advertise a higher CFM at a specific pressure because the air compressor is capable of delivering the rated CFM as it goes from 0 PSI to 90 PSI. For example, let’s say you have tires that run 85 PSI. You purchase a portable air compressor rated for 3 CFM @ 90 PSI, thinking it will do the job. The problem is, the air compressor might be able to supply the 3 CFM of air flow at 45 PSI, but as the pressure increases the volume of air flow decreases. It can supply 3 CFM of air flow at say 0 to 45 PSI, but at 90 PSI the air flow decreases, so the air compressors labors trying to add a few pounds of air to the tire.
6) Actual CFM @ 90 PSI
So the real question is, what is the actual CFM delivered @ 90 PSI? If your RV tires require a specific CFM rating @ 90 PSI you more than likely won’t get it based on the advertised CFM at a specific pressure. What this translates to is, it will take longer to inflate some larger RV tires, if the compressor is in fact capable of inflating the tires at all. So when you go shopping for a portable air compressor, the key is to try and find one that can deliver about 4 real CFM, based on the real HP rating @ 90 to 100 PSI.
7) So what air compressor will get the job done?
With all of that said, the article would not be complete without mentioning portable air compressors that can do the job. Our motorhome tires require 90 PSI for proper inflation pressure. As an RV educator, and RV owner I struggled for answers and a solution to finding a small portable air compressor capable of inflating the tires on our RV.
Then VIAIR reached out to me, asking if I was interested in reviewing the 450P-RVS portable air compressor. I told them I was, and after I researched the product I discovered VIAR manufacturers a line of portable air compressors made specifically for RVS. VIAR has four RV Tire Inflator models, so if you see “RVS” in the model number it was designed and tested to be RV safe and capable. After testing the VIAR portable air compressor I was absolutely sold on it.
Watch my VIAR product review video
Mark J. Polk
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3 thoughts on “Polk’s Top 7 – Selecting a Portable Air Compressor for RV Tires”
Thanks Mark enjoyed your info. Ordered The Viair 300 this morning, looking forward to not dragging my garage compressor around. Thanks and Happy Easter.
My coach has 22.5 rubber and require 110 pounds. Will this compressor meets this requirements. Thanks,
That is the same size tires I have, and it inflates them.