Testing Tire Gauges
August 2, 2010
Filed under Tech Tips
By now everyone knows about the importance of maintaining the proper pressure in your motorhome tires.
Improper inflation is the most common reason that motorhome tires fail and therefore the subject is a common topic at RV rallies and in Internet forums. Even after you have determined the proper inflation needed for your particular tire and coach combination, you still have to rely on a tire gauge to display accurate data or your attempts at correct inflation will be biased by the inaccuracy of the tire gauge. This holds true for your dinghy as well as your motorhome or any other vehicle you maintain that has pneumatic tires.
Many motorhome owners have turned to tire pressure monitoring systems, which allow you to monitor the pressure of your tire while on the road, and we highly recommend those systems as the very best and most comprehensive monitors available. Since they operate all the time, they allow you to monitor your tire pressure as you drive. Most of them even feature alarms that can alert you to an out of spec condition. If you are still using a traditional manual tire pressure gauge to check the pressure on your motorhome, dinghy or other vehicle, it is a good idea to make sure you are using a gauge that is accurate enough for the job at hand.
Most Class A motorhomes have 19.5- to 22.5-inch tires that call for tire pressures in the range of 75 to 120 psi, while Class C motorhomes run smaller tire/wheel combos that typically run pressures in the 50 to 85 psi range. Before selecting your ideal tire gauge it is important to know the maximum pressure that you want to read and choose accordingly. If you are going to measure a maximum of 50 psi you have various options available to you as opposed to someone trying to accurately read a 120 psi tire.
Our goal in this article was to gather a large sample of tire gauges that are available at various retail stores and test them against a calibrated digital gauge and report our findings. In order to very accurately test and compare our gauges we turned to Intercomp Racing Systems, which was kind enough to loan us a digital tire pressure gauge that is certified and calibrated (NIST traceable) to 0.4 percent of the gauge range. That means it is accurate to 0.6 psi and it displays down to 0.1 psi. In our testing we found that even in repeated measurements of the same tire it never varied from its 0.1 psi reading until we had tested the tire enough times for it to lose 0.1 psi of pressure. This gauge sells for $329 and is the gauge of choice among many professional automobile racers who rely on extremely accurate tire pressures.
In order to compare all the gauges, we inflated a 22.5-inch motorhome tire to a series of pressures (30.0, 40.0, 80.0, 100.0 and 120.0 psi) and used the calibrated gauge as the standard to obtain the exact pressure targeted. Not every gauge in our test was capable of the higher pressure measurements so we tested them to their individual limits.
By using a large volume 22.5-inch tire, it allowed us to test it numerous times before losing enough air (volume) to change the reading by 0.1 psi. After getting the tire pressure exactly to the set point using the calibrated gauge, we then used a gauge and recorded the pressure. Each gauge was tested three times at each set point. Then to ensure the pressure had not dropped (even by 0.1 psi), we rechecked the tire again using the calibrated gauge after every test was completed. Although this is not exactly a laboratory test, it is very representative of how a typical owner would use a gauge and therefore we feel confident that it’s a true test of the gauge’s accuracy.
Our testing includes digital, stick and dial gauges. The dial and stick gauges do not have the resolution to display less than 1 to 2 psi of pressure. So naturally they are not going to be as precise as a digital gauge since their scale (or our eyes) is not capable of displaying small increments. For those gauges we simply recorded the pressure in the range that it was capable of displaying, which was usually in one- or two-pound increments. If the reading fell between two increments we split the difference. This doesn’t mean that dial or stick gauges are less accurate, just that they are less precise.
For motorhome tires, we consider acceptable accuracy to be within 2 psi. If you are obsessing over 1 or fewer psi you should know that ambient temperature at the time of the measurement can affect the pressure by that much or more, so it is best not to waste too much time or money chasing that degree of accuracy. It is important to check your tires cold each day before driving as is specified by every tire manufacturer.
All dual rear-wheel motorhomes require a dual-foot air chuck in order to test the backward-facing valve stem, and most of our 120-plus psi rated gauges had this dual-foot design. The gauges that display maximum pressures of 100 psi or less usually do not feature this dual-foot design, so make sure you select the right gauge for your needs.
After reviewing the data for all of the gauges tested, including all of the ones I could dig up around my personal motorhome and various tool boxes, it is clear that all of them are fairly accurate. The worst one read 4 psi high at 40 psi (Slime #2020-A) while another showed 3 psi low at 120 psi (Slime #2021-A). Some gauges were consistently at reference pressure or below, such as the Tire Minder digital pencil. Others were constantly over the pressure, which is less desirable. Many of the gauges were within our 2 psi target and some of them were within 1 psi of reference pressure.
A tire pressure gauge seems like a pretty simple device and on the surface they all seem about the same. We thought the same thing until we gathered more than 20 models and started testing tire pressure with them. Since we used each one so many times, patterns started to develop and so did personal preferences. The obvious difference in the digital models and mechanical stick or dials is the fact that they have greater resolution/precision in their ability to display down to 0.5 or even 0.1 psi. They also require batteries, with some models using four batteries. The mechanical dial and pencil gauges are simple and never need a battery. Of course, they typically only read in 2 psi increments which in our estimation is good enough, assuming you have an accurate gauge.
There are a few features on a tire gauge that we feel are important and the first is a peak hold display. Due to the odd angles often needed to read a tire it is critical for the gauge to hold that display long enough for you to read it, even after you remove the gauge from the valve stem. This feature is commonly called peak hold and we can’t recommend any gauge without it. Another important, though not critical, feature is a bleeder valve. If you over-inflate a tire, this button allows you to bleed some pressure while the gauge is connected to the tire until you obtain the exact pressure desired. We noted those gauges that have this feature in our ratings.
Though we didn’t really intend on declaring a winner in this article, one gauge in particular stands out in many ways — the Milton S-976 Service Gage. First was its accuracy: It had no error at every pressure we tested it at. It is also made in the United States and priced at a reasonable $14.99 at Northern Tool + Equipment. Lastly, it’s built like a tank. If I were ever stranded in the woods with any one of these gauges and forced to defend myself against a wild puma, the Milton 976 is the tool I want in my hand. It is a mechanical pencil gauge, so make sure that is acceptable to you before you purchase it.
If you have poor eyesight you may prefer a digital model with a large readout such as the $27.95 digital unit from Global Trucker. It has the largest display of all the handheld digital units tested, not to mention it had 0.3 psi or less error from 30 to 120 psi. Another product that stood out to us was the Snap-on Blue Point Tire Gauge and Inflator for $87.50. Yes, it is expensive, but it is intended to serve as a compressor-mounted filling device and it too displays amazing accuracy. Plus it allows you to fill the tire and read the pressure at the same time.
If you are searching for a tire gauge for your dinghy, motorcycle or other vehicle that doesn’t use dual rear tires, then any of the car-specific gauges we tested should work fine. As you can see, all of them maintained accuracy within our 2 psi standard. Two of our favorites were the Accutire digital gauge (from www.palmbeachmotoring.net) and the Slime digital tire gauge No. 22019. Both were within 1 psi of the reference gauge and the Accutire gauge even goes up to 150 psi and includes a durable rubber boot and easily replaceable batteries.
Perhaps the most important lesson learned is that most gauges sold today are accurate enough to ensure your tires are properly inflated. Now it is up to you to do your part and check the tire pressure on your motorhome every day before driving. You could save yourself a lot of grief and ensure you obtain the maximum life possible from your tires.