Changing the diameter of your tires from the original tire size affects the accuracy of the speedometer.  You can easily find the correct speed using this formula.

All you need is the original tire's Overall Diameter (O.D.) and the new tire's O.D.  If you don't know what the original tire size was, or what the O.D. was, you can still find out by looking up your car in our store, or by calling one of our vintage tire specialists. 

The formula to use is: 

(New Tire O.D. / Original Tire O.D.) * Speedometer MPH = Actual MPH

 So, for example, if I look up my 1956 Chevy in the Coker Tire Store I find that it originally took 670-15 tires.  According to the Coker Tire Store this tire has an O.D. of 28.58", which gives us our first measurement, the "Original Tire O.D."

 Next we need to find the O.D. of our new tire.  Based on the Tire Conversion Chart, the radial replacement tire is a P205/75R15, which the Coker Tire Store shows has an O.D. of 27.11", giving us our second measurement.

 So, lets say we are going down the freeway and our speedometer shows we are going 60 MPH.  Lets plug in our measurements into the formula:

(27.11" / 28.58") * 60 MPH = Actual MPH

First, divide the new O.D. by the Original O.D: 

27.11 / 28.58 = .95

All that is left is to multiply this times the MPH shown on the speedometer, giving us the Actual MPH:

.95 * 60 = 56.9 MPH

So, based on this example, changing my 1956 Chevy from the original 670-15 tires to the P205/75R15 Coker Classic Radials means that when my Speedometer shows 60 MPH, I am actually going approximately 56.9 MPH.