I just got a fancy tire pressure gauge from Griot’s Garage. It has a digital readout and measures in tenths. The gauge clamps to the tire valve so you can use your hands to bleed air from your tires. The instructions say that it is plus or minus 1% accuracy at 76°.

This morning the garage temperature finally got down to 76° so I checked it out. Working back and forth between the TPMS and the gauge, I bled the air out of my tires until the new gauge said exactly 32 lbs in both front tires and exactly 39 lbs in both back tires. That made the TPMS read 30 lbs in both front tires and 37 lbs in both rear tires.

I am guessing the TPMS reads 2 lbs lower that what’s really in the tires. I can live with that because I usually run a few extra pounds of air in my tires anyway.

I had a similar experience. I used a Topeak digital gauge that reads to 0.5 psi to set my tires to 31/38, and I think the temperate was around 75F (car in garage, not driven for 14 hours). I think I saw 29/36 on TPMS, which matches the same 2 psi low as you see. When I look at the delta-from-normal (in TPC menu), I saw +1 all around, which matches my gauge. The manual tells you use the +/- display to adjust, and never to adjust based on displayed pressure. I don't understand how the +/- display can be accurate while the current pressure display seems off.

The TPMS in my car also reads roughly 2 psi below my guage on all 4 tires, with the tires hot or cold.

Greg

Mine too...