I don't doubt he can hit 40mph by himself. I've done 38mph easily 10x in a row during sprint workouts. I think another way to film it would be for the car to stay next to him, film him perfectly synchronized with the car when he hits 40mph, then pan over to the speedo. Or have a helmet-cam and film the computer on the bike.

What I want to see is Boonen hitting 47mph by himself!!