I will be using an older (90's) Trek 920 singletrack mountian bike with road tires on it (1.25" wide - can you say low rolling resistance?). I'm expecting a harsh ride (but no harsher than a regular road bike with the same tires), good top end, and real good gas mileage with these tires. I'm going with the 43cc Mitsu engine and Staton chain drive kit. I will gear it TALL for better top end (or lower rpm's at 30mph) and plan on pedaling it up to 10mph or so to get started (and get a partial workout, take load off drive componenets, get better mileage, etc). The bike speaks for itself. It's a cro-mo frame and fork...Trek quality. It was an expensive bike even back then. I will definitely go with the beefed up rear rim from Staton (48 spoke, 1" rim) though.
Warner