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Thread: tyre circumference

  1. #1
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    Default tyre circumference

    Hi All..

    I have a vectra c with 225/45/17 tyres on it.. this works out to 198cm tyre circumference.


    however my car is set to a tyre circumference of 202cm in the ECU (i assume due to the OEM tyre size of 215/50/17 that the car ships with)..

    couple of question i cant get my head around:

    is the speedo showing the correct speed?
    am I going faster or slower than the displayed speed on the speedo?
    is my fuel consumtion effectivly showing better of worse than it should be?

    many thanks
    steve
    PS: I know I can just change the setting in the ECU, but I'm intrested in knowing the answers..

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    Regular Member Penfold101's Avatar
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    It's showing you as going faster than you actually are, so your fuel economy will show better than what it actually is (I think) as you're using the same fuel but not going the same distance per revolution.

    Try a tyre shop online as they usually have calculators to show you the exact figures and percentages...

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    Regular Member chris1804's Avatar
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    sorry to hi jack im getting some 17's fitted on my car but the car has 16's on atm will i have to change the system to 17's or wont it affect it much?

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    Regular Member robengland9's Avatar
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    I've always thought of what happens when wheels get changed with bigger rims etc...rpm will not change but this must alter speed and mpg readings if ecu is not changed to correct wheel dia... i'm interested to know also...?

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    Regular Member Penfold101's Avatar
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    If you find a tyre website with a calculator you can put in the new tyre size and it'll show you the difference. Anything over 2% larger or smaller starts getting serious errors in it.

    Vauxhall list specific tyre sizes in the handbook for the various sized alloys, under the tyre pressure section. There are also options in the ECU to change the size that the ECU thinks is fitted, because the speedo readout is based on what it THINKS is fitted.

    To explain: the ABS sensor records the rotational speed of the wheel using the number of revolutions, and using that figure the ECU works out the car's speed based on the diamter of the tyre fitted. If you have a different tyre size fitted, then it won't be reading accurately on the speedo. A bigger wheel will put the speedo lower than you're actual speed, which is illegal, and a smaller wheel will make it read higher, which is fine.

    Any questions...?

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    Heres a tyre size calculator, should answer your query. http://www.etyres.co.uk/tyre-size-calculator.htm

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    Thanks Penfold.. Thats what I was looking for..

    Below is a couple of snap shots of the op-com screens for tyre size options that can be fitted to a 2007 vectra c

    Hope this helps..

    Steve



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    Regular Member lee gsi's Avatar
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    I use this one as it shows how far out the speedo will be too: www.kouki.co.uk/utilities/visual-tyre-size-calculator

    The difference in the 2 tyre sizes is marginal. A new tyre with full tread and another one on the limit will be further apart with the same size tyre fitted.

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    I just worked out if you had 17inch dia and changed to 18inch that would be a difference of 3.8%.... so if you was doing 70mph on your speedo reading you could realy be doing 72.66.... dont think it would be enough to trigger a speed camera but if you was going just over 70 then the 3.8% extra might just catch you out.

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    Regular Member Penfold101's Avatar
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    Quote Originally Posted by robengland9 View Post
    I just worked out if you had 17inch dia and changed to 18inch that would be a difference of 3.8%.... so if you was doing 70mph on your speedo reading you could realy be doing 72.66.... dont think it would be enough to trigger a speed camera but if you was going just over 70 then the 3.8% extra might just catch you out.
    What sizes did you put into it for the tyres...?

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