Air conditioning in the car reduces fuel efficiency because it takes energy from the engine to run the internal cooling unit. When it is extremely hot outside, it totally makes sense to use the air conditioner, but cool your car the same way you do your house -- by maintaining a temperature around 74 degrees rather than blasting the coldest possible air. Also, park in shady spots whenever possible, and start by flushing hot air out of the car by opening all the windows when you first get in.
ANDREWS MCMEEL SYNDICATION