I'll admit that I'm slowing down to save gas, but I always do it in the outside lane. I've got a 2008 Scion Xd that's rated for 27/33 (automatic). By (a) starting off slower and (b) keeping it between 55-60 on the interstate, I can average 42-43 MPG for the tank (it resets itself every time you fill up). When starting off, I remember something I read about pretending your foot is on an egg on top of the gas/break petal. Don't push down too hard too fast and if you see a red light ahead, take your foot off the gas.
I'll go 55 on the interstate and have everyone speeding by, but I don't mind. Most of the stretch I'm on is rated 65/55 so the trucks are supposed to be going 55 anyway (which they don't). By the time it gets to the 70 MPH zone, it's starting up a big steep hill so the trucks are slowing down to 45 by the time they reach the top anyway due to gravity. Down the other side and I'm ready to exit.
I'm also lucky... the town I'm in had a new gas station come in and they're having price wars. It's down to 2.58 already for regular :)