I was burned on tire pressure once. The temperature dropped and so did my mileage. I couldn't figure it out but when I checked my pressure it all made sense: colder air condensed the air in the tires and deflated them. But it works the other way too: check the pressure when it starts warming up. Anymore, I check every week - more if I suspect anything.

I run 39/37 because I want to run higher than spec but still a little woozy about over-inflation.