Back in 1959 on my BSA 10-speed bicycle, with sew-ups and a strong tailwind on new asphalt and a steep downgrade, I kept up 70 mph for several minutes--according to the guys in the MG sportscar who were right behind me the whole way. My highest speeds on knobbies recently have been no more than 38-40 mph, but at age 62 and on dirt that'll have to do.