[garys_1k]

Originally Posted by ptatohed

I believe that, so long as the engine is at operating temperature (and cold ambient temperature has little affect on an engine reaching operating temp.), a car is more efficient in colder weather. Heat is truly the enemy for efficiency. The colder the ambient temperature, the more efficient your car is. Cooler, more dense air into the throttle body, reduced coolant temperatures, reduced oil temps, etc. Not to mention if you drive at night, you'll be less likely to use AC, there will be a tendency for fewer cars on the road, you'll make more green lights, etc.

Anyway, thanks for the great tips!

You have any data to back that up? Cold air is denser and causes more aero drag. Lower internal temperatures can trigger enrichment or delay optimal spark timing in a calibration, too.