Holy crap it's scary that he's building his own speedometer like that. GPS based speedos aren't all that accurate. Ballpark, sure, but not accurate. There are also many circumstances under which GPS won't work. In many countries a car without a functional speedometer is not allowed on the road, for safety reasons.
Drivers who want to go fast will go fast - for them the gauge is only useful so they can slow down for speed traps.
Not having a speedometer shouldn't make a sensible driver suddenly very dangerous at all.