[Christ]

There's a calculator here somewhere that will tell you based on your frontal area and weight what kind of power you need to keep 55MPH steadily... if you're only using that much power, you're not likely to ever actually hit 55MPH. The calculator assumes excellent specs, like terrain, wind, etc.

You should be prepared to have at least 30% more than is really necessary for the desired speed you have in mind, for any kind of decent acceleration. Also, consider the weight of all components when using the calculator. You're not going to weigh 2,500 Lbs anymore, you'll be closer to 4,000. It takes more power to accelerate and sustain that weight.

The continuous duty rating on a motor is important, but I don't think it's quite as important as the "burp" rating, or the high level that it can achieve, and how long it can sustain it. Often, a lower HP motor can handle making a bit higher HP figure safely for a given period of time, but then will need to be run more nicely to cool back down and run without damage.

That said, you'll be looking for more than 15HP, almost certainly.