I personally just like the 'Helping you Save at the Pump' since it's the slogan of the site and I think gets the point across very well.
'Seperating Fact from Fiction' is maybe what we do, but sounds confusing to me to someone that knows nothing about hypermiling or gassavers.org.
How about just:
I think that's the biggest thing for me with hypermiling. I really don't notice the miles go by anymore and am not agitated at all or anxiously scanning for cops and everything. Just physics and logic and no worries.
Or 'Save Money, Save the Environment, Save your Stomach Lining' Heh.
Wouldn't be able to use it in Texas, can't have anything on the top covering the word "TEXAS". It'd have to be pretty thin and small to avoid getting bothered by local cops. But I could go for having just GasSavers.org on the bottom.