I am restoring a 1967 Mustang, but I'm replacing the motor and trans with a 351w motor and C6 trans. I am also installing a set of 15x7 wheels all around the car. I am running what I think is a standard rear axle but I dont know the ratio because my VIN plate is blank under axle. I'm just wondering what size tires to run on the wheels and if I need to replace the speeometer gear in the trans or recalibrate the speedometer itself. Thanks, Psycho

 

The way to recalibrate the sppedometer is to change the gear that goes in the back of the tranny. Play with this calculator to see if it gives you the answers you are looking for. You will need to know what size tires you now have on it, and what size tires you hope to put on it to see if any recalibration needs to take place. It would also be helpful to know the gear ratio of the differential.
http://www.4lo.com/4LoCalc.htm

Here's some more info you might find helpful. http://www.mustangmonthly.com/howto/38118/index3.html