I've driven from Austin,TX to Madison,WI a few times (~1250 miles) in cars 25-33mpg avg, and what you claim sounds like BS dude. I stopped more than a couple times, to be safe whenever I got to a half tank and stretch.
My 1250 mile trip was 20 hour drive strait though, your claim would be closer to 28 hours non stop.... Really?
The only car off the top of my head that has 800 miles per tank is a newer Audi A8 diesel, and at $100k, a rare car for consumers. Most vehicles on the road I would say are under 400 miles per tank.
More details on what you drove, what speed, etc.