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      06-04-2010, 04:47 PM   #7
richard in NC
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Quote:
Originally Posted by solgain@mac.com View Post
Memphis, is my math so wrong? The web site calculated the difference between the 17" and 19" as 0.9 %

So according to my interpretation it would be say at 100 MPH 109 MPH (17" OEM wheels and tires against 19" wheels and tires). What am I missing here?

Thanks for the follow up
If you increase from 17" to 19" you need to dcrease the aspect ratio (45 to 30 in the example above). If you don't do that, the diameter will change and the speedo will be off. You will go faster than indicated, and the GPS was correctly stating that. However, BMW speedo's are notoriously optimistic. Probably 2 mph at 60, allowing a 2mph increase in actual speed to actually be more accurate.
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