I have a Greenfield E2000, ride-on. Its power output is 22HP and does 13Kph flat to the board. It has ample power to be geared up for higher speed. Oh yes, after the big rains in January 2011 I bogged the mower twice. Even with the diff lock on. Had to get off and push

My Grandson has had a little motorcycle since he was 7. I clocked him at 44Kph. The motor is about 150cc. Its too small for an adult to ride.
So those V8s are very much under stressed. I'd like to try one out myself.
Awesome but not practical.
Cheers