That might ne possible, or perhaps you didnt fill the tank as full as when you started? I got 57 mpg on a 40 mile round trip once. I was flat out hypermiling though. Best tank average was around 49. Both of those are with 100% stock car and 40 psi in the tires. My usual average is 34-42 mpg depending on how I drive, summer/winter blend fuel, ethanol %, etc.
Just out of curiousity, what kind of gas mileage do you get with the geo metro?