I just got a 91 z28 and I've found a problem. My speedometer is off, it thinks its going 20mph more than it really is, and the rpm on the dash might possibly be off too. I'm not too sure. Before i got it a 5 speed racing transmission was added so I'm sure that's the problem, but how do i go about fixing it??

Also, I want to add a supercharger to it. Any comments on what the best one would be??