Originally Posted by Haags
This drives me nuts. I have had my 06 for two years, and I have to add air to the tires every time the temperature drops, then usually another 1-2 times during the winter months. I assume this is normal, but I've never had a car with pressure sensors so I have no point of reference. The warnings come on when the pressure gets to around 30-31 psi and I have to add to get it up to 35. I've had to do the spare 4 times in two years, which seems excessive and is a real pain in the *** because of the location of the stem on top of the spare.
This is perfectly normal. You're just not accustom to having TPMS. If you have friends/family or other vehicles, this will hold true as well. For every possibly 8-12 degree drop in temperature, air compresses in, thus your PSI drops as well. A good practice is to add about 2-3 PSI more per tire, when the tires are cold, and your not freezing your butt off too much outside, during the day. This way, when the weather gets colder overnight, the 2-3 PSI you added will eventually level to the manufacturer's setting (generally 35 PSI, which is what my XK requires). As it gets warmer via by season, then you'd just have to leak a few PSI out. In regards to the spare, the tire does not do any pressure changes, so with temperature changes, it'll also go down and down.
The best solution is shoot your tires up with nitrogen. Nitrogen does not change its state per temperature changes. So regardless, your tire pressure will be the same regardless, unless you have a puncture or leak on the tire.