I think 66 cuz of the little bit of extra torque helps alot when u need it going up a tall hill or going off road, or down a muddy dirt road. lol. Pedaling down dirtroads that have lots of ruts, is not fun. I tried it and 10 ft later, i was puffing and couldn't pedal. Using the engine helps wonders. It dug a nice deep trail behind me. U could hear the engine at about 10mph just pumping out the hp. lol. Pure awesomeness. Some of my friends brought the idea of me taking my bike mudding. lol Sounds like fun, just gotta know whats under the surface of the mud first, and as long as it isn't as deep as it gets mud on my engine. lol Gotta keep it purdy! jk