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Wheel/tyre size differences from std fit- inaccurate speedo??

trekbiker Oct 9, 2005

  1. trekbiker

    trekbiker Member

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    I'm currently running 265/30/19 on a B6 avant , which i have been told by an experienced employee of ATS are 10mm bigger in overall circumference than the standard fit of 205/55/16 or 235/45/17 which would normally be standard fit.

    these wheels were fitted by an Audi dealer from new. I have 2 questions:-

    1. Would the speedo read slower than i'm actually going and by how much(is there a calculation?). would the mileage also be affected as my MPG, which i have worked out by brimming the tank against the mileage(supposedly) done?

    2. Would a smaller tyre, say a 235/35/19 be better? /ubbthreads/images/graemlins/noidea.gif

    Any help please!
     
  2. AndyMac

    AndyMac Moderator Staff Member Moderator

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  3. trekbiker

    trekbiker Member

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    thanks Andy

    So my gearing has definitely been altered. What i need to know now is the speed variation for the MM measure difference!
     
  4. AndyMac

    AndyMac Moderator Staff Member Moderator

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    Well it's the same as the percentage difference in rolling diameter.
    i.e. your setup is shown as out by 1.58%, so when your speedo hows a speed og 100mph you're actually doing 98.42mph. As all speedos are 4-5% out the other way anyway it will actually make the speedo more accurate.
    I run 225/40/18's which gives me a 5mm increase in rolling diameter and according to my Road Angel my speedo is now only 2% out (shows faster speed than actual).
    Those tyres must cost you a fortune, aquaplane city!
    You'd be better off with 245/30/19 's which will be 2mm out i.e. less than the difference between new tyres and worn ones, but personally I'd go for the recommended (& cheaper) size of 225/35/19 to avoid tramlining & surfing.
     
  5. simon_golf_tt

    simon_golf_tt Member

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    Are the 265's on the front as well as the back? I've noticed that some of the US guys run 265's on the back and 225/235 on the front. They usually have 1" wider rims on the back too - 8.5" front and 9.5" back. Some even run this on Quattro and don't seem to have any problems. I bet the 265's look excellent but I'm not sure how it effects handling - how does it feel?

    As Andy says - new tyres will hurt!
     
  6. trekbiker

    trekbiker Member

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    [ QUOTE ]
    Well it's the same as the percentage difference in rolling diameter.
    i.e. your setup is shown as out by 1.58%, so when your speedo hows a speed og 100mph you're actually doing 98.42mph. As all speedos are 4-5% out the other way anyway it will actually make the speedo more accurate.
    I run 225/40/18's which gives me a 5mm increase in rolling diameter and according to my Road Angel my speedo is now only 2% out (shows faster speed than actual).
    Those tyres must cost you a fortune, aquaplane city!
    You'd be better off with 245/30/19 's which will be 2mm out i.e. less than the difference between new tyres and worn ones, but personally I'd go for the recommended (& cheaper) size of 225/35/19 to avoid tramlining & surfing.

    [/ QUOTE ]

    mucho gracias.. it need 2 new boots soon so i may change the lot and go for something more common and cheaper. Michi pilot sport at trade was going to be £500 a pair!. It handles Ok, but i have only had it for 2 months and we aint seen much rain here in Mk. IT seems to manage the monopoly of roundabouts here fine, though not as compliant as my TDI Q i had last year(this one's FWD)
     
  7. AndyMac

    AndyMac Moderator Staff Member Moderator

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    Ouch! Goodyear F1's 225/40/18 are only £230 a pair
     
  8. domcj

    domcj Member

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    235/35/19's i run there are about the right size fromn what I can make out speaking to a few people from various garages
     
  9. AndyMac

    AndyMac Moderator Staff Member Moderator

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    235/35/19's are even further out - 15mm bigger.
     

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