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Thread: Speedo Accuracy with Rim Change

  
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    Speedo Accuracy with Rim Change

    Hey guys, when you change the rim size, i understand that the speedo will become less accurate, unless you reduce the profile of the tyres to equal the total circumfrence of the stock wheels, so i was just wondering, what you can do to "recalibrate" (if at all) the speedo if you fitted bigger rims without reducing the rim profile

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    tech2 from vauxhall can change it mate
    also what astra do you have as some cars get the speed from the gear box

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    I got a Mk4 Astra 5dr hatch 1.6 LS

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    It's never gonna be that accurate anyway - standard is'nt that accurate

    If it was a big problem, there would be much more info out for solutions...

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    Cos i based the issue on this, or is it exaggerating the problem ?
    http://www.rochfordtyres.co.uk/tyrecalc.asp

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    what year is your car as i am sure the speed on that 1.6 will come from the gear box

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    Quote Originally Posted by dannysport18
    what year is your car as i am sure the speed on that 1.6 will come from the gear box
    Its on 02 Plates

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    How Tos . Parts For Sale
    The two most life changing experiences you'll ever have. Finding Jesus, and having a baby.

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    Surely it doesn't matter where the speed is calculated from.

    If the gearbox was reading 40mph at 3000rpm in 3rd with regular wheels and you go for a wheel tyre combo that has a bigger circumference then the speedo is reading 40mph at 3000rpm in 3rd you're going to be going faster.

    For example changing from 205/55 R16 (standard SRi wheel/tyre combo) to 215/50 R17 that is an increase of 2.36% in circumference, so when the speedo says 40mph your actually more likely to be doing 41+.

    I know this example is only a tiny bit, but they are what are calculated as 'a good fit' (close comparison) if you went for a wacky size, and were doing 70mph on the speedo, you might actually be doing 75-80+

    I'm not sure if this kinda thing is tested in MOT, I'm sure someone will be along shortly who can tell ya.

    But if you are set on a wheel/tyre combo and it's wrong, Tech2 can sort it for £50ish.
    0-60 in 6.5 secs and 48mpg- winner!

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