I have a base 2007 with TPMS and 17" wheels. Last summer the system and my tire gauge gave the exact same readings. I put winter wheels and tires on last fall (with Tire Rack TPMS) and the readings were a little below my guage. I did switch the computer to correct 17" winter "tyres". I now have the original summer tires back on and switched back to 17" summer tyres on the computer. The PTMS reads a very consistent 3 pounds under the "actual" pressure. I am defing actual as the same reading I get from my old guage, a new guage, and a gas station guage.
Any thoughts or fixes? I'm still under warranty but not sure if it might be operator error, and not the boxster.
Thanks