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I just switched back to my stock wheels this past week for the winter time. I had always suspected that my 17's read a little fast but I was never sure. I now know for sure that they were about as spot-on as they could be. At 60mph with both sets I am at 2100 rpm, and at 70 mph I am at 2500 rpm in both sets. How can the speedo read the same if the 17's are 8/10's of an inch shorter in overall diameter? The only possible explanation I have thought of is that the Kuhmo's have a much stiffer sidewall so maybe they do not flex as much when put under weight as much s the continentals do. That seems a little far-fetched however. Anyone have any thoughts?


235/45/17 Kuhmo 712's = 25.3"
215/60/16 Continentals = 26.1"
 

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im no expert in wheels, but 8/10 of an inch seems pretty minimal, i wouldnt expect there to be much difference there.

another thing maybe that the speedo just seems to be in the same place. but i really wouldnt expect any speed difference with that minimal of a size difference
 

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What are the aspect ratio's of both sets of tires, I went from stock 15" steelies running 205/65/r15 to SHO rims runnign 225/50/r16 with NO change in the speedo
 

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With bigger tires your car will go faster but the speedo will still read the same, its a function of mechanics. The speedo drive is on the tranny so it doesn't care what size tires you put on, your RPM vs MPH will remain the same. But the cop with the radar gun will see the difference. You can go find a dynometer. If you have an OBD11 '95 and later rated vehicle, the speedo can be reprogrammed to read correctly.
 

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Discussion Starter #6
Bob that is an interestering take. So you are saying that the speedo is dependant on rpm, and is a set ratio. Changing overall diameter of the tires wont affect the speedo AT ALL? But in reality it will affect my speed because of the size change. That makes complete sense, but I have never heard that before. I always understood that the speedo was controlled by a sensor on the axle somewhere and that is why if you changed the overall diameter of the tires it would throw off the speedometer.

And the tire pressures are the same for both tires.
 
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