There are special friction type bicycle tires sold for just this purpose. I never researched this so I don't know where you can buy these tires, but they have a thicker tread and a flat smooth surface almost resembling a dragster slick. The weed eater motor is mounted over the back tire and drives a small neoprene wheel that is brought into contact with the rear wheel with a ratchet mechanism controlled with a cable selector at the handle bars. The throttle is controlled by a conventional throttle twist grip. I've heard claims of over 200 MPG at speeds of 12 MPH.
Lamborn's Miniature Engines