The Stolen Tools

Tools were stolen from a construction site. Max Mouse found footprints and a tire track, leading him to believe there were two thieves - one walking and the other riding a bike. Even though Slylock Fox believes both tracks are related to the robbery, he suspects there was only one thief. Why?

Slylock Fox Puzzle

The tire track can not be from a bike. The tracks were left by one thirf pushing a wheelbarrow, full of stolen tools.