Just came back from a trip with the Vibe. On 1 tank we went 423 miles! It took 11.6 gallons to fill ( 36.4 mpg) It is hard to do but if you can keep the speed below 75 these cars reward you with this. I am thinking with lower speeds a 500 mile range might be possible. Need a long low speed ( 45 - 50 mph ) trip and a little luck and who knows. I have seen as much as 40 mpg on my car. Anyway, for what it is worth.
Congratulations on the good mileage. The only short highway trip I've taken netted me just under 32 mpg with my XRS (still not broken in at the time). I don't care if I could get 100 mpg -- I'm not driving 50 mph on the highway. I barely drive that slow in town.
XRS 6-speed, Cosmic Blue, 17's, AEM Short Ram, Hidden Hitch