Upgraded to 18" Do I need to Fix the Speedometer

M

MDZ

New member
Hello, I have a 2003 Z4 2.5i, I just switched the stock 16" wheels to a set of staggered 18" Velocity Motoring wheels. I was wondering if I needed to change a setting in the computer, so that my speedometer will read correctly or if since most cars read a couple high from the factory, if I was good to go. Thanks for the help
 
On my previous Z (2004 - 3.0), I changed from 17 to 18". The result was that the speedo was more accurate then ever before. I verified via GPS before and after, and at 70 MPH it used to be 65 on GPS, and after was 68.
 
Not necessarily, it depends on the circumference of your tire. Usually an 18 inch tire runs a lower aspect ration than the 16 inch tires does, i.e.: 16 might have a 235 x 45 series tire while an 18 inch tire might have a 235 x 35 series tire. The diameter of the wheel and tire combination would be the same even thought the wheel is 2 inches bigger. FYI, this tire size does not equate the same just used it for an example. Tire Rack has a program to put in old size and new size as does this site, http://www.1010tires.com/ then tire calculator.
 
As Above....

The Speedo sometimes get better with the change of size, but really the rolling radius should stay the same, the wheel gets bigger but the tyre wall gets thinner.

A good wheel place should know and allow for.

Cheers

PaulN
 
Thanks guys you have been very helpfull, according to the link you gave me it said:
Speedometer Difference: 0.685% too slow

So I guess it will just correct for approx 1% of the factory 3%.

Does anyone know if there are any other reasons but the speedo to have the information set correctly in the computer? I was thinking that the traction control system might uses that information.
 
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