Shopsmith Forums

How do you verify the accuracy of a tire gauge?

I have 4 different tire gauges. Two of them came from Harbor Freight. One is in-line with the tire inflator, and a separate 6" gauge came with it as seen in the image below.



I don't know where the other two came from, but one is another 6" gauge marked Made In Taiwan and the other is a "mini" 3" gauge that has no "Made In" origin markers on it. I usually keep the "mini" in the saddle bag on my bike.

Every single one of them has a different reading when checking the same tire. And the difference is quite large. Or it seems so to me. About 12psi separate the two extremes with each gauge almost evenly spaced apart within that range.

So how do I know which one (if any of them) is right?

12psi over what 'range'?

I would bet the in line inflator(more rugged construction) but they ALL are dependent upon spring accuracy.

FWIW the barrel type must be used such that the indicator rod does not overshoot the spring/plunger position. Make sure when taking a measurement(pressure applied) that the indicator rod is flush against the end of the plunger.

JPG40504 wrote:12psi over what 'range'?

This was three days ago when I checked, but I think it was 32 at the low end and 44 (or 46) at the high end.

JPG40504 wrote:I would bet the in line inflator(more rugged construction) but they ALL are dependent upon spring accuracy.

Well, it is Harbor Freight and I wouldn't quite call it "rugged construction" if you know what I mean!

Also on the gauges that do not have a spring pulling the plunger back in you usually need to apply the gauge to the tire stem and then with the gauge still under pressure gently push the plunger back in until you feel resistance. Often the plunger shoots out too far on some gauges just from momentum.

Ask a reputable tire dealer to verify them for you. They will (should) have calibrated master gages that they use to verify their working gages that the techs use. (Don't be afraid to ask them how they know their gages are accurate).

FYI - In California, it is going to be law very soon (this summer I think) that garages, dealerships, oil change places etc. verify and adjust tire pressures whenever a vehicle is in for servicing. They are mandated to use calibrated gages.

This post was triggered by the "Shopsmith Deals-- not such a deal" thread but since this is just a side story I thought it belonged here.
We opened our first store as a vacuum cleaners and small appliances retail store downtown in 1974. I carried pretty much all of the major brands that were available at that time and we serviced "everything".
About a year later another vac seller from a nearby small town moved his store in just a couple of blocks down the same main drag. He did a lot of bragging about how he was going to put me out of business. He also was at times, well, a little less than honest and spent a lot of time bad-mouthing us to people that shopped there. The guy was a little wild-eyed and always uptight and hyper like one of those bug-eyed little barky dogs.
I knew from his ads which units he was selling and with the Eureka line we had mostly the same models on the floor. Actually Eureka was his only vac line but he sold some other unrelated items. We carried a full line of Hoover and Sunbeam and several others as well as a couple of lines of commercial vacs.
I had bought a large lighted changeable letter window sign that had a row of flashing colored bulbs all around it. It was a good deal and got us noticed a lot. He watched our store like a hawk and if I ran an ad he ran an ad. No big deal. He would send family members in to price shop us. I guess he didn't know that I knew who they were. One day I kind of started messing with him. I would start chatting with those family members and feed them a line about how many Eureka vacs I was selling. The numbers were a tad inflated. OK, maybe inflated a lot. There was one base model vac that we sold with very little mark-up just to have something to advertise at a price. We sold quite a few since I didn't believe in bait and switch (but was always happy to "sell up") and if that was what they wanted we sold it to them. I always kept a big stack of them at the back of the store so everybody had to at least walk through the store to get to them. One day I listed that model on that changeable sign for around 25% under my cost for 3 or 4 days. I;m not sure that any of my customers even saw that sign and I did not sell a single one at that very low price but the guy down the street sure saw it... It about gave him a stroke... I found out that he had even called the Indy Eureka office and "screamed" at them over the phone because he was convinced that they were selling them to me for less money than they would to him.
A lot of his big talk kind of faded at that point and he finally retired a couple of years later. He turned the business over to his son and his son and I became good friends and even worked our businesses together a tiny bit to our mutual advantage for a while. We would swap inventory at times, especially parts. We would also split shipments now and then to get a better price. His son never tried to "drive me out of business".

mickyd wrote: FYI - In California, it is going to be law verify soon (this summer I think) that garages, dealerships, oil change places etc. verify and adjust tire pressures whenever a vehicle is in for servicing. They are mandated to use calibrated gages.

Mike, on a kinda related note, there's a place in Missouri now that (thanks to the Federal Stimulus program) opened a FREE tire filling station where you can replace the air in your tires with Nitrogen. According to the claims:

...Based on Environmental Protection Agency estimates, if 10% of St. Peters residents use nitrogen in their vehicles' tires, they would save about 68,400 gallons of gasoline, saving money and emitting less carbon dioxide, reported the St. Louis Suburban Journals....

tom_k/mo wrote:Mike, on a kinda related note, there's a place in Missouri now that (thanks to the Federal Stimulus program) opened a FREE tire filling station where you can replace the air in your tires with Nitrogen. According to the claims:

Pardon my apparent ignorance, but how does 100% nitrogen provide any 'savings' over 78% nitrogen and 22% other gasses?

100% Nitrogen??? Please explain to me how the normal air is removed from the tire before it is inflated with 100% Nitrogen. Costco (among others) have been filling with Nitrogen for some time, but when they tell me that I should not mix air from my compressor with the Nitrogen in my tires, they could not explain to me how the Nitrogen that they put in my tires displaced the air that was already in them. To do so wood mean that they wood either have to start out with tires that are sucked to a completely collapsed state before inflating them with Nitrogen or some kind of valve that will allow the escape of air as Nitrogen replaces it. Costco does neither. IMHO, every tire that is "filled" with Nitrogen still has more than 50% regular air mixing with it. Please, someone educate me.

BTW, the Nitrogen that Costco inflates your tires with is just normal air filtered to eliminate all but the Nitrogen.

[quote="a1gutterman"]100% Nitrogen??? .....IMHO, every tire that is "filled" with Nitrogen still has more than 50% regular air mixing with it. Please, someone educate me. ...[quote]

Don't have an answer for you Tim.
all I know is when I bought tires at Costco for my wifes car, they filled with nitrogen. That was about a year ago and to the best of my gauge, they haven't lost any pressure.

The tire folks at Costco told me that if I ever have to add air to tires - come in and they will re-fill with nitrogen at no cost.