Cav's sprint on the track is about 41 mph. Top track sprinters can hit 43-44 mph on the track, but Cav is nowhere close.

On the road, random air turbulence makes it harder to achieve the same max speed as on the track (unless that air is a tailwind)! So, no freaking way Cav could do 48 mph (or 41 mph) by himself on the road. There is no need to believe this or not believe that, there are plenty of evidence of him on the track.