replaced speedometer driven gear with recommended 40 tooth and still not accurate...

Close to it. I don't remember the exact numbers. Speedometer is accurate to about 12mph, then the faster the speed the higher it is off. Doing 50mph (per gps) the speedometer was reading 62mph. Had the rear end rebuilt a couple of years ago and was told it's a 3.91 gear... I was thinking about putting a 41 T in it but sounds like I need even more?
Thanks... I'll get there. I did find out today however that one tooth equates to 3mph on the speedometer. Talked to a tech guy at A and A Transmission in Indiana. Mancini didn't know...