I might be getting to wrapped up in this tire pressure thing. Bought a new digital AC air compressor device( which, of course may not be that accurate). Set for 42 front/39 rear. Tires “cold” overnight in garage here in FL. Reset and drove a few miles. The results : Front 39/Rear 35. I have been known to try to remove fly feces from pepper. Help!
Your experiences appreciated. Nick

 

One of the curious things about air is that it expands and contracts with changes in temperature. A law of physics and nuthin' gonna change it. I'm guessing that the random temperatures in FL do not remain stable from hour to hour.

 

My tire guys (Discount tires) do any work, (new rotation) and I ALWAYS have to check and adjust afterwards. Doesn't drive right on the way home. I got an electronic gauge and it was off, not very accurate, checking each tire twice there were different reading up to 2, 3 pounds. Did a little research and went with the Accutire, tested to be the most accurate. At Amazon. It works.

 

Thanks. I just went for a 20 mile spirited drive. Tires heated up and I am within a pound or so on each tire. I sometimes can get too wrapped up in minutia. Love my JCW. Much nicer/ faster than my last ‘11 JCW.

 

Keep in mind that when you compress air, it heats up. Also, if you park the car outside, the side facing the sun will be hotter and the tire pressure will be higher.

 

I am accustomed to 1 to 1.5 pound difference between the TPMS reading, and my old school (and high quality) Longacre guage.

If TPMS says 32.5 for example, the Longacre will measure that as almost 34 pounds.

With a continuous source of tire pressure data during daily operation of the car I have learned that from cold start in the morning at 65 degrees, to autocross conditions at 80 degrees (after a run, and on a day with a co-driver) the front pressure will rise 20%!

I've seen a 31 pound tire expand to 37 pounds running down I-81 in Virginia on a warm day with a 75 mph cruising speed.

As previously stated, I've seen tires gain 1.5 pounds just from being parked in the sun.

But I have learned to take the TPMS numbers seriously, although unsure about the calibration - the numbers are precise and consistent.

Cheers,

Charlie

 

I would not get too tied up in the difference between the mass produced TPMS sensor and your hand held pressure gauge. Just pick a pressure that feels good and provides even wear for your driving style and call it a day.

Not a perfect 'rule of thumb', but fairly good is 1psi per 10 deg of tire or ambient temp change. Up here where temps can swing from below zero to 100+ deg, that equates to ~10psi pressure swings just sitting still. On the track, seeing 5-7psi pressure change isn't uncommon. My optimal 'track' pressure usually ends up in a 'soft' street pressure post event, where I am usually pulling 2-3psi out of the tires after my first track session at an HPDE. In the winter time I am usually adding ~5psi to the tires to keep them in my preferred range. Heck, going around a good size on/off ramp, you can see your outside tire pressures rise a bit, but they will even back out as you travel straight again.

 

I have two air gauges that are pretty accurate (checked them against each other and another gauge) that I always use. The electronic air pressure that shows up in MINI connected is never the same. I go by the gauges as they are a direct measurement (as opposed to the indirect tire radius/rotation time method used electronically). As someone said earlier, air pressure is a crap shoot anyway, so I check once a week in the morning when it is coolest and if they are within a pound either way, I call it good. Car tires are pretty forgiving anyhow.
On my motorcycle, I am more obsessive as the pressure can be the difference between life and death.

 

The sensors used in TPMS tire stems directly measure air pressure. However they vary in accuracy because they are primarily meant to warn against blowout rather than for maintaining proper tire pressure.