Tyre sizes

    Hi every one, especially those who supported me on my last request for advise.


    Still having trouble selecting wheels for my 850 CSI only now because of technical issues.


    Wheels selected are R&H AS Speiche, and they are 8.5Jx18 front and 9.5J x 18 rears.
    These were offered by the supplier as best fitting taking into account wheel offsets.


    Now using the tyre calculator i am unable to get the % difference less than 2% for the rear and 2.78% for the front.


    The recommended is +/- 2.5%


    My question is as follows.


    Is the difference of 0.78% between front and rear critical or should it be closer to ensure abs / asc sensors do not misread between front and rear?


    Is 2% -2.78% acceptable as a difference from the original wheels and tyres on the 850CSI or should i aim for a tighter tolerance band?


    Your advice will be greatly appreciated.


    Thanks to all

    Tyre Sizes


    Aologies
    Furher to my request for advise i can give the tyre recomendation as proposed by the wheel calculator.
    Tyre combination #1
    285x35x18 = 2% difference from original wheel
    255x35x18 = 1.2%difference from original wheel
    How ever the difference between front and rear is 0.7%


    Tyre combination #2
    285x35x18 = 2% difference from original wheel
    255x40x18 = 2.8%difference from original wheel
    How ever the difference between front and rear tyres is -3.2%


    The reason i ask, is that the difference between front and rear on existing BMW wheels and tyres is 0.1%using calculator, and i cant help thinking that the ratio/difference from fron to rear is more important with ABS/ASC than than being 2 miles out on the Speedometer?


    Correct me if i am wrong please.

    If you are less than 3% front to rear then the asc will work as advertised no matter what the difference in overall diameter from stock.


    If you are within 3% of stock then the Speedometer (operator interface) and the Speed Signal (computer interface) should be within tolerance.
    BUT-If you are at 3% I would verify the speedometer accuracy if possible. It isn't too good thinking that you are OK and then getting your picture in the Mail :(

    It could be that the purpose of your life is only to serve as a warning to others. Mine sometimes feels that way.


    Terry


    1995 840Ci
    1991 850i
    Woodland Washington USA

    285/35-18 rear and 245/40-18 front here.


    I have had 255/35 and 265/35 on the front at one time or another with 265/35 on the rear also with no unusual side effects.

    It could be that the purpose of your life is only to serve as a warning to others. Mine sometimes feels that way.


    Terry


    1995 840Ci
    1991 850i
    Woodland Washington USA

    Tyre sizes


    Terry Y
    Thanks for prompt reply.
    Can i pick your brains again?
    Is the front wheel size important other than it has to be within the 3% for the ASC to function correctly.
    I only ask this as the car is rear wheel driven and do they not govern the speedometer reading throght the diff and gear box ratios, or is the speed calculated via the rpm?
    just curious


    Thanks again

    Speed is calculated from the pulse sensor in the differential.


    Wheel slip for the ABS and ASC is calculated from the 4 individual wheel sensors. A difference of more than 3% front to rear is interpreted as wheel slip.


    So if any wheel is going slower or faster than the others by more than 3% it will trip the ABS on braking or the ASC in normal forward travel.


    Given that ASC controls several engine functions and the individual rear brakes on ASC +T equipped cars, wild moments might be found when it creeps above some electronic threshold at speed :shock:

    It could be that the purpose of your life is only to serve as a warning to others. Mine sometimes feels that way.


    Terry


    1995 840Ci
    1991 850i
    Woodland Washington USA

    If it's any use, my Owners's Handbook lists "permissible" tyre/wheel combinations.


    If you use 18" rims it specifies 285/35 rears and 245/40 fronts.


    I wrote a small Excel spreadsheet to do accurate tyre/wheel calculations (which is very easy to use) and it didn't quite match these sizes, but given my experience with insurance companies I've stuck to the manufacturers specs.


    I have been in an Eight with 19" rims, but the ride was too harsh for my taste - I have 18" which I believe gives the best compromise (and doesn't affect insurance premiums or claims).


    Hope this helps.

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