lunarhighway's idea would work very well. It would be a bit expensive...
Stepper Drive
Motors and Cables
PLC counter for the VSS
DC powered PLC
The control method would have to be set up in speed domain ranges. The motors would be in one position up to, say, 35 mph, and then lowered for up to 55mph, and lowest above that.
I
think the PLC counter would work for the VSS input.
Oh, and depending on the how many pulses/sec the VSS might send, a lower end PLC (D0-05DD-D) could be used and the counter might be eliminated.