Re: theoretical top speed?
It depends on top rpm. I tried to answer that in posts #4 & #5. If you can get the motor up to 6,000rpm, then you would get 58mph, but real world I would expect 5 or more mph less with wind resistance. If you could get the Briggs to spin at 7,000, you could achieve 67mph, but I would minus out at least 5-10mph for wind resistance. Also to get your Briggs up to 7,000rpm would likely take a performance cam, billet rod, and stiffer valve springs.