Where's your math coming from? At 3600rpm, 26" tire, and 14:1, I'm getting 19.89 MPH on the speedcalc program I use... And smaller tires will be slower yet.
Believe me, I burnt out several neurons trying to figure out any way I could use one. Unfortunately, Every story was the same. I'd have to feed it around 10,000 rpm to get a satisfactory result. And there's no chance I'm going to have standard b*ll**it driveline parts turning 10K. That's a disaster waiting to happen. FYI, that meant overdriving the tranny output about 1:3 on a jackshaft into the axle.