<center><img src="http://pictureposter.audiworld.com/17158/TirePressureGauges.jpg"></center><p>I have both a mechanical and digital gauge. When outside temperatures were about 32 deg. F both gauges showed the same tire pressure. Yesterday, with temperatures around 70 deg. F they were about 2 degrees apart. The mechanical gauge showed 44.5 PSI while the digital gauge showed 42.5 PSI. Both gauges were exactly repeatable - I tried each one 3 times, each time with the same results.

I am curious if the discrepancy could be due to thermal expansion in the mechanical gauge. If so, does a 2 PSI delta seem reasonable with a temperature increase of 40 deg. F? The digital gauge calibrates to 0 PSI before each reading. Has anyone experienced this before? Do I need to buy a 3rd gauge so that I have a simple majority when I check tire pressures?

<center><img src="http://pictureposter.audiworld.com/149/accu-gage.jpg"></center><p>I'd be more apt to trust the digital gauge. Or you could get yourself a higher quality mechanical gauge.

Accu-Gage makes a good one at a decent price. You can get one at the link below.<ul><li><a href="http://www.carcrazy.net/bin/find.cgi?p=356">http://www.carcrazy.net/bin/find.cgi?p=356</a></li></ul>

I bought two AccuGages and found that they differed from each other by more than 2 psi. Each guage was consistent; they just didn't agree. I called the manufacturer, which sent me a third guage to try. It agreed with one of my original ones within 1 psi or so, so I returned the other one.

AccuGage told me that the disaggreable guage was probably out of calibration. They will calibrate their own guages free if you have doubts, though I didn't go that far with it as I doubt 1-2 psi makes much difference considering all the other factors, notably temperature, that affect tire pressure.

Works great. I ordered wheels from Ronal. Gary said they had 36 lbs in them. I measured them, my gauge read 36. A friend had a digital gauge, so I put that on there. Same reading.

Most digital measurement systems are actually inferior to the mechanical systems they replace. This is because the digital systems are made to be cheaper, not better. Look up mechanical bathroom scales some time.

It is definitely possible to make digital systems that work better, but it isn't nearly always the case.

It took years for CD players to sound better (be more accurate) than good analog equipment, and they make millions of those a year.