I read this somewhere recently, and if I could remember where I would certainly tell you... But alas, I can't recall, so I apologize for not sharing my source.
When merging onto the freeway, your car will accelerate faster and more fuel-efficiently if you switch off the AC for the 10-15 seconds it takes to accelerate to freeway speed. It also works well when merging into traffic while going up a hill (which I have to do all the time in my neighborhood). Try it and tell me if it worked for you!