Basic tire pressure question

Need some input on tire pressure.

I purchased one of those fairly “cheap-o” air compressors from Amazon that has a digital gauge and lets you preset the PSI and it shuts off automatically. But I’m getting really weird readings.

I set the compressor to 36 PSI, and it fills it up until it reads 36.5 PSI. They say they do this to account for the air being lost when unscrewing the valve. My truck reads 37 PSI. Makes sense enough.

The odd part is, when I use an old school stick type PSI gauge, it says 40. This variation is consistent too. Happened on both front tires. Rear tires I fill to 34, compressor fills to 34.5, truck reads 35, but stick gauge reads 38.

So, which one do I believe? The compressor, the truck, or the gauge?

 

I would say the readings on the air compressor and the TPMS on your truck are probably the closest. Most of the digital readouts are +- .5lbs. So those two are essentially reading the same within their accuracy limits.

 

My tire guy (yes, I have a tire guy) said you can buy 5 new guages and get 5 different readings. I bought a cheap guage and go by that; I'm not concerned if it's off a pound or two.

 

Pick one. Use it.

The "accuracy" of these gauges will be all over the place. Some will be as good as 3%, others 10%, some will be based on error of reading others will be based on error at full scale.

The problem is you don't know without detailed testing.

I'm not surprised the readings don't match.

FWIW, look at the sticker in the driver's door for the recommended tire pressure. IIRC, its 29 psi.

 

I usually find the tpms reads a little low, but they are close. The stick style ones are usually pretty off in my experience.

Just use the truck read outs and you'll be fine.

 

I use a cheap textron digital gauge. Had it awhile...seem to be very accurate. Its always within 1 lb of the trucks dash gauge.

 

Thanks guys. I figured the truck and compressor readings were the closest. I just always thought the stick ones were the most accurate. But I guess I got the cheapest one. Lmao

 

I actually had a question about the sticker on the door panel. It says 29 psi, but I don’t have the tires or wheels on it that the truck came from the MFG with. I actually took the wheels off of my 2020 SR5 and had them put them on my 2021 Sport (some premium predator package wheels). I did the chalk test and came up with 36 for the front and 34 for the back.

Got any insight on that? I’m not a very knowledgeable mechanical person, but I’m always happy to learn.

 

The "accuracy" also depends on the Maximum pressure of the gauge.

0-50 psig will have tighter error range when compared to a 0-100 psig.

In a past working life, I used electronic pressure gages. All were +/- 0.1% Full Scale accuracy. The lowest range was 0-50 inH20, the highest range was 100,000 psig. I had error ranges from +/- 0.05 inH2O to +/- 100 psig.

Suggest to use a gage with a full scale of 0-50 psig for the tires. Whatever brand or type. Pick one, use it.

 

29psi is cold, so whatever the lowest ambient temp you drive the truck at.

I usually recommend 30psi for cold months and 32-35 for warm months.

I ran 28 but didnt care if the tire light came on.

 

Does that 29psi mean if I had the factory wheels and tires on it? Or any wheels and tires?

In my previous response I mentioned I kept the wheels that were factory on my 2020 SR5.

 

Seems the lower the tire pressure when using those stick gauges the less accurate they become. You get down to around 12 lbs for those lawn tractor tires and some of those stick gauges won't even register. You have to buy a special low pressure stick gauge if doing that. Imo, just get one of these texton digital tire gauges and be done with it.

 

You could start with a quality US made gauge.

It can be hard to find, but they're out there for around $35.

It's easy to buy Chinese goods, but when you do, your supporting the CCP and very poor working conditions.

**MOD EDITED: memario1214

 

Is it this guy by any chance?

 

Neither.
Believe the cheap Chinese product?
Or believe (likely) Chinese stick gauge?
As has been noted above, buy 5 gauges and get 5 different readings.
Also noted, buy a stick gauge with the lowest usable range, typically 0-50 PSI.
I wouldn't expect digital gauges to be super accurate either (where were they made?).
Keep the tire pressures consistent. Watch the wear. If they wear consistently in the middle, the pressure is too high. Adjust accordingly.
JMNTBHO

 

As an old saying goes, "A man who has one watch always know what time it is. A man with two watches is never really sure."

I'd go off of the truck. Not because it is the most accurate as much as that's what the computer will go off of. The is some variance between my Topeak gauge and my truck. I believe my Topeak is more accurate, but there's really no way to know. (Especially since I can't rely on the dealership since they love overinflating my tires.)