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Thread: 205/50/16 to 225/40/18 speedo?

  
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    205/50/16 to 225/40/18 speedo?

    swapping my 16s for 18s what have 225/40/18 tyres on them

    Currently my speedo reads +3mph on the 205/50/16s
    e.g when it says 30mph im actually doing 27, 60mph is really 57.

    Im just wondering if anyone could tell me how far out my speedo will be with the new 18" wheels+tyres

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    supersterl's Avatar
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    http://www.tyresave.co.uk/tyresiz2.html

    this will tell you that actual difference between the 2 wheels..
    Drives a 53 plate Arden Blue Zafira A Gsi 2ltr turbo...

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    Windrew's Avatar
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    you know you can program the clocks to the correct wheel size!
    ARDEN BLUE GSI

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    Ye i already worked this out :-) the 18s diameter will be 1"/26mm.. bigger with tyres.

    Was wondering if some smart fellow coule tell me how much this would knock my speedo off by

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    Quote Originally Posted by Windrew View Post
    you know you can program the clocks to the correct wheel size!
    i did not :P how do you do this?

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    Quote Originally Posted by Gparker View Post
    i did not :P how do you do this?
    not sure if op com can do it but i know tech 2 can, its just a case of changing the code index to suit the size wheel/tyre you are fitting, saves it reading wrong all the time, as you milage wont be correct too
    ARDEN BLUE GSI

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    ahh n/m i worked it out speedo will be 4.2% out so at 60mph i will actually be doing 62.5mph.

    As my speedo is -3mph out i will be doing 59.5 when my speedo reads 60

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    AOC has it's own app for this telling you difference in size and speed http://www.astraownersclub.com/tyresize.php
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    from a few threads iv seen about the same situation i think youll need 215/35/18 tyres on a mk4 for the speedo to be correct.

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    As Ben has said above, go for 215/35 stretched tyres on 18's if you plan to lower the car. On a coupe, 225/40 will scrub the arches.

    Just had a look on the AOC calculator,

    Going from 205/50/16 to 225/40/18 means you will be doing 72.95mph when your speedo is saying 70mph.

    Going from 205/50/16 to 215/35/18 means you will be doing 69.57mph when your speedo is saying 70mph.

    Advice I got yesterday is that you want the difference in terms of size to be within 2.5% of the original size. Having 225/40 means the overall wheel will be 4.22% bigger, going for 215/35 will mean the wheels will be -0.61% difference.
    Astra sold ... replaced with a Skoda ... soon to be replaced with another Vauxhall

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