If you're familiar with the car you just need a tach. Or even just your ears. But that's on the road. The only reason you really need a speedo is the police--otherwise, who cares? On the track, average speed (lap times) are good enough. But even corner to corner, there's no way your revs won't be an accurate enough indication of speed. Remember: KMH and MPH are arbitrary! If in some sport someone really needs the exact speed for some setup optimisation or what have you, that will be readily available to him via telemetry and/or speed traps. Where that information is not available, it can't be that big of a deal.
P.S. I was not aware real F1 cars had speedometers . . . do they indeed?