We've had the mini cycle vs mini bike discussion here before. If the knowledgeable folks who've participated in this thread thus far had assumed he was talking about 8" wheels, they wouldn't have responded as they did.
I have a Tote Gote with white wall 8" Shinko SR550's which are DOT-rated to 60 MPH. It not a mini bike either.
Also, gear ratio and speed charts become fairly useless when computing those kinds of speeds. If the engine can't maintain the RPM either because you blew it up, or because it doesn't have the torque to manage a gear ratio sufficient to maintain that speed. We can be theoretical all day long. My current build with six inch wheels, and a modified 4HP flat head Tecumseh will theoretically do 49.