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Discussion Starter · #1 ·
Just wondering is there any way to calibrate the speedo to ensure its still reading properly? My car is running on 18" mv2 style with 225/40 tyres on them but going by the original spare the standard wheels i think are 15" 205/60.

I worked out that the speedo would be reading about 13% slower than actual speed going by wheel circumference, is there any way to count this in the speedo? Or is it just a case of bearing in mind your going faster than is being let on to you? This has already affected me i was cruising along quite the thing not really thinking bout this and the thing and got pulled by an unmarked cop car:frown So will already always be bearing it in mind was just wondering can you correct it?

One other thing on the side, how do you tell if an alloy is genuine bmw or not?

Cheers guys

Dmc
 

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Legendary BFuk Member
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When you car is doing 70 mph with the 225/40 18's on your car will be travelling at 71.14mph.

Speedo calibration can be done but it is a specialist job:thumbsup
 

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Discussion Starter · #3 ·
Ah right i see why i was wrong now. In hindsight 13% is a bit rediculous. Guess the reason i got pulled was cos in the beemer when im used to a fiesta you dont feel your speed at all, just thought one factor might be the speedo being out. My bad lol

Anyways, cheers for help :thumbsup

Dmc
 

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You allowed for the increase in wheel size but you forgot to calculate the difference in the tyre side wall. Many people think that when you get bigger wheels and tyres the overall circumference is so much larger.

The larger the wheel diameter, the thinner the tyre side wall gets to compensate. Any wheel and tyre change must really stay within +/- 2.5%:thumbsup

It is easy to look down at the speedo of a BMW and think S**T I better slow down:hihi
 
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