Inflate the spare to 15-20% over the regular running pressure in the other four. That won't hurt it, and it's easier to let air out than to force it in .[JWR's Comment: If you do this, for safety be sure to prominently tag it "Overinflated!" add also pack a tire pressure gauge.] Since people rarely check the spare for air, the 15-20% adds a margin. Use your spare as part of a regular 5-tire rotation maintenance plan (every 5,000-to-6,000 miles is about right); that keeps its wear about the same as the other four, and adds at least 20% to the overall life of the set of tires over a 4-tire rotation schedule.