Letting gravity do the work. I was thinking about the doing a go-kart race down Shudehill the other day. It would only be a short sprint, however. I’m sure there are other hills near Manchester that would offer better challenges and less traffic to stop, and for the truly insane there are a few really steep roads in the Pennines with sheer drops off one side.
The San Fernando Valley Illegal Soapbox Federation meets on the second Sunday of every month to charge downhill on wide American roads in races of twenty or more karts. Which is cool.