Since September 1, 2007, all new cars sold in the US are required to have a Tire Pressure Monitoring System (TPMS); the European Union will make a TPMS mandatory in 2012. These laws are intended to help drivers detect under-inflated tires. The systems are calibrated to trigger when a tire is at least 25% below recommended pressure. Driving on under-inflated tires increase tire wear, use more fuel or electricity and can be unsafe in some cases.
US Roadsters automatically check the tire pressure of all four wheels and warn you when the pressure gets too low. You have two options: the standard high performance and more energy efficient usage pressure and an alternate comfort mode. By default, the Roadsters are set to the recommended limits of 30 pounds per square inch (psi) for the front tires and 40 psi for the rear tires.
After over seven months of driving, I received my first warning on a cold day. Tires typically lose on to two psi for every ten degree drop in outside air pressure. Surprisingly, my tire pressure was quite significantly low before the TPMS warned me. My front right tire was at 23psi and my limit was at 30psi. My right rear tire was at 31psi, and the limit was set at 40psi.
The TPMS system was quite accurate and was exactly the same as my manual pressure gauge, and the gas station attendant’s more sophisticated gauge. He kindly filled all four tires with air even if I clearly was not going to buy any gas there that day.
With all of my other gas powered cars, I routinely take them in for oil changes and really do not pay much attention to tire pressure. I would prefer that the TPMS warned me earlier about the low pressure. The tire pressure monitor is very easy to use and I should easily be able to check the pressure manually on a more routine basis.