Re: How can one figure out the "correct" tire pressure?
I see a couple of issues here. One is the disagreement between TPMS and a manual gauge, another is what the dealer called the correct pressure versus what the door panel says, and finally you asked what is the optimal pressure.
Originally Posted by ThomWT
First, from what I read on this forum, ignore the discrepancy between the TPMS and the manual gauge. Use an accurate manual gauge to set the pressure. The TPMS is really for monitoring a low pressure or flat tire situation, it's not really designed to be an accurate method to set or monitor pressures. Whenever any shop touches your tire pressure (or lug nuts), always check or re-torque them yourself. They just don't take the time to set them accurately.
Two, ignore what your dealer said. Dealers and shops typically overpressure tires. If you ask them about it, do you expect them to say "we made a mistake?" No, they will tell you that they did the right thing. This "nature of modern tires" thing is just B.S. to get you off the line. The dealer should, in theory, set the pressure to what the factory specifies. If they don't, any reason they didn't is just an excuse.
Three, how do you determine what the optimal pressure is. Well, that depends on how you define optimal. The factory recommendation is what they believe is the best compromise between safety (e.g. creating understeer), handling, comfort and mileage. Do you want to optimize mileage? Go slightly higher than the factory recommendation. Optimize the ride? Go slightly lower. Get rid of understeer or optimize handling? Well, now that gets tricky... The best way is on the track with a tire pyrometer. Other than that, change the pressure in one or two pound increments until you dial in the feel and balance (understeer/oversteer) that you like, while monitoring (via chalk or white shoe polish) the wear patterns to make sure you don't roll over onto the sidewall.
’08 Cayman S, 6MT, PASM, Sport Chrono, Sport Seats & Steering Wheel, 19” Carrera Sport Wheels, PCM, Bose, Xenon, plus some other stuff not worth mentioning