Just to use as an example....

The Stock wheel + Tire diameter is 688 mm (27.08")

The aftermarket wheel + Tire diameter is 655 mm (25.78")

I figured this out to be....

Stock tire turns 745.76 rev/ mile
Aftermarket tire turns 783.38 rev/mile

With the aftermarket wheels and tires, will it make the speedometer inside the car show a faster or slower speed that you actually are going??? By How Much???

Does anyone know the formula to calculate this so I can apply it to many different applications???

 

Smaller tire turns more RPM making the speedo read faster than you are going.

By how much? Get someone to pace you and find out.

 

This is correct and he's racking up incorrect mileage as well. It's not difficult to do the math on a percentage basis.

If you take the tire diameter difference from stock you can figure the speedo error. I did this when I changed my 245-50/16 tires to 255-50/16. There was a 3% difference in the speedometer and odometer. At 100 mile per hour on my speedometer I was actually going 103 by the radar.

 

In this case, the speedo and odometer error is:

688mm / 655mm = 1.0504

So the error is 5%. If the the true speed is 60mph, your speedo will say 60 x 1.05 = 63mph. If you drive a true 1,000 miles, the odometer reading will increase by 1,050 miles.