I don't know about going more than 10MPH on a riding mower, but locally there are a lot of guys who have modified riding mowers for a "junior" tractor pull. My neighbor, across the road has been working on one for the past couple of years, and he started going to competitions this year. I don't know how much power his rig has, but when he fires it up, it sounds like a small dragster. He has some rather large "AG" type tires on the rear, and earlier this year I watched him practice down in his field...he was pulling an old pickup around with the rear brakes locked. He created some serious trenches in his field.