How does tire size effect the speedometer?

I have 15" wheels on my speedster and riding down the road at 50 mph it shows 3K RPM. Is this right? Now if the speedometer is showing 50 but the car is realy doing 60 mph then that would be more like it. Any info on this will be helpful.

Mike

Mike Hall:
It is not the wheel size, but the tire diameter that affects the spedometer reading. The speedometer is set up to assume a specific (stock) tire diameter. Each revolution of a wheel is assumed to produce so much footage (mileage). If you change the tire diameter (e.g. going from 165/75 to 235/70) you change the footage per revolution and thus the miles per hour reading. (Even same size tires from different manufacturors have slightly different diameters.)
Use the 'measured mile' posts on highways to determine your actual speed at and indicated 60MPH. The you can ask the guys at a speedometer shop for different speedometer gears to try to compensate, but it is likely to be a cut and try process.

Generally, it is easy to measure the overall height of the tire and then compare it with the height of the stock VW. Of course, taller tires mean fewer RPM per foot of travel. Shorter (low profile) tires mean higher RPM per foot of travel. To check your final-drive differential ratio you can:
1. Jack up the rear of the car.
2. Put it in high gear.
3. Mark the location of the valvestems on the body.
4. Mark the crank pulley location.
5. Rotate the engine crank pulley and count the rotations as a ratio of the tire rotation.
Once you know your final drive ratio you have an idea of what is the right height tire.

my speedo is set up like a stock vw where the driver's side front wheel spins a cable which is connected to the speedo. 165r 15 tires are 25.4" in dia and rotate about 816 times per mile. this produces accurate mph readings. if your tires are bigger they will rotate fewer times in a mile, causing the speedo to read a slower speed then the car is actually traveling. there used to be a site that would let you calculate and compare the effects of tire size changes and if I can find that I'll post the info. You can roughly compare sizes by looking at tirerack to see where your tire size compares with others in their tire specs.

check this out; www.miata.net/garage/tirecalc.html