Beginning in '06, the Prius has a direct tire pressure monitor system which monitors the tires independently. In other words, if all four are low, the system WILL warn you.

 

That's good to know. From what I can find, the TREAD law allows either a direct monitor system (with sensors in each tire) or an indirect system (which monitors wheel rotation). The requirements are slightly different for each system, but only requires the system to detect a drop in tire pressure of 25-30%, with no requirement to sense if all four tires are low. Direct systems can easily be more accurate than that, and detect all four tires independently, so its nice that Toyota switched. Hopefully its calibrated so that it still detects small drops if you're running your tires at a higher than recommended pressure.

 

Our pressure warning light comes on when the temps drop below freezing here in Central Virginia....and then go off when the weather improves. Probably I need to bring the car to the dealer as I suspect the pressure is just low enough that the cold lowers it enough to trigger the warning.

The cold weather also has dropped my mpg, which were 54 consistantly. I am now getting 50.7. Can't wait for Spring!

 

I may be off by a bit, but I think the official ruling is any vehicle (of a certain type; there ARE exclusions, though mostly in heavier vehicles) manufactured for sale in the U.S. after September 2006.