Speedometer off a bit

[Cannonizer93]

Hey guys just got my car tuned and the speedometer off a bit 41 mph=35 , 51=45 80mph=65mph. Just wondering if maybe somebody put gears in the car and what ratio it could be. It's a 99 cobra

 

[DiabloGarrett]

jack the rear end up, mark the driveshaft and spin the tire 1 revolution. Then count the number of times the driveshaft spins = gear ratio. That will give you a pretty good idea

 

[99Blkcobravert]

Are you running stock size tires?