Quote:
Originally Posted by zuhl
How is that possible that you are going slower than your speedometer with tires that are considerable larger than stock? I mean, it just does not compute. I believe you, but it does not make sense how that could happen.
For every one turn that the wheel sees, it is registering the distance travelled as the circumference (3.14 * 29.5 = 92.67 in.) of your factory tires. Your circumference (3.14 * 32) is larger than that by 8.4%, so at 60mph on you speedo you should be doing roughly 65 mph in reality. My error with 31.4" tires is about +7.5% as measured by two different GPS systems. You should have to get smaller than factory tires to accomplish what you have reported here.
Was your speedo dead on before the tire swap?
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Exactly. Think about two cars equal in every way from the factory. Swap out the tires on one for larger tires, with a larger circumference. Take those two cars and have them do the EXACT same speed - the one with the larger tires will register LESS revolutions per any distance X than the one with original tires. A larger circumference means to cover X distance, you have to turn less revolutions. If LESS revolutions are measured in the driveline for any distance X, what do you think the speedometer in the car with larger tires is going to do if it never got told the tires changed size?