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  1. #1
    Join Date
    Mar 2005
    Location
    Northampton
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    1,103

    Tyre rating & size

    The time has come to change tyres, the Bild tyre survey listed on another post was very helpful, I think I'll be going for the Continental premium contact 6 which performed well. Preferably I'd like something with white lettering on the side but getting these with a decent performance is tricky I'm not going down the Avon route as I'm not convinced they are the best tyres, they are costly, and they start off with relatively shallow tread depth.

    The question I have is what speed rating ? V and above or W and above ? (It's a 4.6 Rv8 ).

    The other Question is slightly different: The speedo under-reads by 5-6%, so if you travel 100 miles on the stanav, the odometer registers 106. This is a pain when following tulip style route maps etc. Would taking the rear profile up from 60 to 65 (or 70) correct the error ? They are 235 60 R15.
    David (AKA Firestarter29)

    Lead, follow, or get out of the way.

  2. #2
    Join Date
    Apr 2004
    Location
    East Leake, Nottinghamshire, England.
    Age
    55
    Posts
    9,537
    If you increase the rolling diameter then the reading inaccuracy will get even worse. As for a set distance traveled the wheels will of turned less revolutions. If you decrease the diameter this would held but I think the aesthetic look would not be a good look.
    Martin

    351W 416.7 HP and 466.8 ft-lbs

    Running Mega Squirt and Edis 8


  3. #3
    Join Date
    Mar 2010
    Location
    Hertfordshire
    Posts
    934
    As above if your Speedo is reading lower than actual speed by 5% you'd need to reduce the rolling radius by 5% to increase the axle/gearbox speed by 5%. To get that with just a tyre change would mean coming down from 235/60's to 205/60's. Probably wouldn't look right

  4. #4
    Join Date
    May 2008
    Location
    Cambridge
    Posts
    136
    Increasing the rolling diameter means for a set distance the wheels will turn less revolutions. This in turn will mean the odometer reads less for that distance, which would appear to be what you want to achieve. You describe the odometer over reading, not under reading. If your odometer reads 106miles now for 100miles travelled, then you want a 5.66% reduction in number of revolutions per distance. Your tyre current nominal diamter is about 623mm, you probably want a diameter about 658mm to correct your odometer reading, so a 67.5% sidewall at 235 section would be desirable... Can you get a 235/65 or 225/70 in the tyre you want? Either would be ok, the latter closer to your desired diameter.
    --
    Richard
    GD Mk3 LS6

  5. #5
    Join Date
    Mar 2005
    Location
    Northampton
    Posts
    1,103
    Clearly I have confused myself quite successfully. Richard, thanks for your comments, it was the sort of thing I was looking for, I'll go away and work out which way I mean to do it.

    Any thought anyone on which rating to go for ?
    David (AKA Firestarter29)

    Lead, follow, or get out of the way.

  6. #6
    Join Date
    Apr 2004
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    on the edge...almost falling off
    Age
    57
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    8,533
    Dave, why not just adjust your speedo??

  7. #7
    Join Date
    Oct 2006
    Location
    Toton, Nottinghamshire
    Age
    46
    Posts
    877
    David,

    It'll take a very small reduction in tyre profile to reduce the rolling circumference by 5%. I think you'll struggle to get the right combination of tyre width and profile to get the circumference bob on. If you normally pump your tyres up quite hard then reducing by a couple of psi may give you somewhere nearer to what you desire.

    I know of some Cobra owners using tyres as low as S-rated. Personally I'd feel happier with V's at least.

    Have you looked into North Hants tyres any further? They do a V-rated 235/60R15 with white lettering. I'm tempted by them for my next pair but I don't want the white lettering on my tyres. Hopefully it's only on one side and the tyres aren't directional.

    Russ.
    Joint Area Rep - East Midlands Region
    1984 DAX Cobra
    Rover 3.5 V8 with Holley carburetor.

  8. #8
    Join Date
    Aug 2016
    Location
    WELLINGBOROUGH
    Posts
    111
    Hi,
    I think it is easier to re - set the speedo. Use the tyres which you want - do the calcs and reset the speedo dip switches.

    sceptic

  9. #9
    Join Date
    May 2008
    Location
    Cambridge
    Posts
    136
    V or W, either should be acceptable. V = 149mph, which is about where the theoretical redline on my car is, so the minimum I should use. I have a 4.6 RV8, lt77 gearbox (0.87 5th gear IIRC) and 3.45 ratio diff and very similar tyres to you, so I'd guess something similar to your car.
    Cheers, Richard
    --
    Richard
    GD Mk3 LS6

  10. #10
    Join Date
    Mar 2005
    Location
    Northampton
    Posts
    1,103
    Quote Originally Posted by scepticwoody View Post
    Hi,
    I think it is easier to re - set the speedo. Use the tyres which you want - do the calcs and reset the speedo dip switches.

    sceptic
    There aren't any dip switches, and last time I sent it off it was away for months !
    David (AKA Firestarter29)

    Lead, follow, or get out of the way.

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