What gear?
 

I have a 79 Trans Am with the 4sp. I took out the 100 mph speedometer & put in the new 150 mph one - rear gear is original 3.23 as well as the trany - can anyone tell me if I need to switch speedometer gear at the trany & what No# teeth that would be? Thanks bunch.

You should be OK, If you did not alter either the rear end ratio, or the Speedo drive gear in the trans, it should be as accurate as the original speedo.

The only wild card is what the driven speed of both Speedo's are. Without knowing that I can't get any more accurate. If the ratio is the same on both speedo's (and I think it should be) it should work as well as the original.