That, as you can imagine, helps improve rolling resistance, compared to the winter months when lower tire pressures result in more drag.
Warm air also affects oil viscosity too. Oil becomes thinner when it's warm, which is fairly academic once the engine is up to temperature, but important when the engine is cold and can spin with less friction--and of course, it takes less time for an engine to reach its ideal working temperature in warm weather.
Finally, some states sell different grades of gasoline in the winter than in the summer. In summer, gas has higher energy content, burns more efficiently, so more of that gas is used to move you down the road, improving mileage.
So there you have it--want to know why your car is getting better gas mileage now than it was six or seven months ago? Plenty of reasons! Just enjoy it while it lasts...