Power(HP) has EVERYTHING to do with top speed.
You need power to negate the drag on the car.

My car has 6 speeds, yet it hits tops speed on 5th and not on 6th. There, I proved you wrong.

Motivation: I have a GPS navigator installed and an Escort 9500i which also has GPS.
It uses the GPS to detect speed to switch between City and Highway modes.
It also uses GPS coordinates to lock-out "fake" signals like that pesky CVS Pharmacy.
Pretty nifty