I was wondering if there is a way to determine what the difference in rear tire size would have on my speedo. I am looking at several wheel / tire combinations and wonder how much a change in tire diameter size before I notice it in the speedo. One combination is less then 1/2 inch larger in diameter.

Chuck...

02, A4, Slp induction, Flowmasters 35th Anniversary and very red

 

First, you'll need to figure out the revolutions per mile for both tires.
To do that, Divide 63,360 (inches in a mile) by the tire's circumference. Circumference is 3.1416 x the tire diameter (overall height).
Say you have 26" tall tires for instance, 3.1416 x 26 = 81.6816.
63,360 divided by 81.6816 = 775.69 revolutions per mile.
Do that for both tires, then divide the New tire figure into the Old tire figure.
That will give you a plus or minus multiplication factor (depending if the new tires are taller or shorter), in decimal percentage form, to figure the rate of change on the speedo.
Easy as pi?

 

Thanks Camaro 69
I made a spread sheet that calculates tire width, sidewall height, and tire diameter from Tire Racks formulas, I will add this to my calculation in determining what I will do when I replace my wheels and tires. The combination I am considering results in just over 1 percent.

V802 Chevy