If you run the tires a little higher (I kinda have mine overinflated at 34 PSI cold), use cruise control a lot, drive the speed limit, accelerate at a moderate pace, you can get some good milage out of them. The best I've gotten to date was 37MPG freeway in my GSR with I/H/E.

Copper plugs might help, if you don't mind changing them every 10-12K miles