The tire pressure monitoring caps are set to a specific tire pressure, which cannot be adjusted. They sell the caps with a number of preset pressures, so you should be able to find one that fits your application. The preset pressure is printed on the top of the cap nice and big so even a station attendant will know how you prefer to inflate your tires.
So I grabbed a set and put them on one of our cars. They go right on, all you do is screw them on instead of your regular black valve caps. This set was calibrated to 36 psi. Before we installed the smart caps we set one tire to 36 psi, one to 30 psi, and another to 25 psi. We screwed the caps on and who would've guessed, they worked! Just like they claimed, the cap on the 36 psi tire indicated green, meaning it was filled correctly. The tire set to 30 psi showed half-green, indicating a tire needing air, and the tire with only 25 psi showed no green at all, telling you that you need air immediately.
I have to say that I was impressed with the simplicity and low cost of these useful accessories. Tire pressure is one of the most neglected maintenance items, and something like this can turn a chore into a no-brainer. As long as you can deal with what I consider to be an ugly appearance, they seem to be great.
