Hmmm... I get that sort of economy when I drive 70 on the freeway *and* there's a kayak strapped to the roof rack (that tends to to screw up the aerodynamics a bit). I've also noticed poor economy when bucking a headwind (e.g. 20+ mph winds while trying to drive close to 70 on the freeway). Exceptional driving conditioins aside, my freeway mileage is MUCH better. I occasionally get 33mpg (I got that on a trip from Detroit to Kalamzoo and back -- just about 300 miles for the round trip).

I'm wondering if you can think of other reasons for your fuel economy.

In the marine world, every additional 300 rpms of engine speed will DOUBLE the rate of fuel consumption, but only push the boat marginally faster. Driving at high speed will kill fuel economy.