Souped up Lawn Tractor

I decided to go from a 6 1/4″ pulley on the rear, to a 3 1/4″ pulley and now the tractor does an estimated 10+ mph. I could still get it to go faster but I couldn’t find a pulley that would fit the motor.

http://www.youtube.com/watch?v=f2czgs8oako&hl=en