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Re: Tyre diameter

"Mike G" <metier@lycos.co.uk> wrote in message
news:13g8419fopimqda@corp.supernews.com...
>
> "David Haggas" <DavidHaggas@compuserve.com> wrote in message
> news:LrmdnWf3Y8j1n5naRVnytwA@bt.com...
>> Apologies if this has been discussed before but why does an increase in
>> width of a tyre increase diameter of the wheel?
>>
>> I am thinking of changing rear 255/35R18 and front 225/40R18 to 225/45R17
>> for better ride characteristics on an 02 E46 sport convertible. Using a
>> tyre size calculator http://www.kouki.co.uk/Utilities/tyresizer.asp the
>> diameter of the 255/35R18 is 63.57cm. The 225/45R17 is 63.43cm so this
>> would be a good replacement where the speedometer is concerned. However
>> if a 255/45R17 is used the diameter shows 66.13cm. I understand the 255
>> is the width of the tyre so why does the diameter and rolling radius
>> change?
>
> As I indicated and forgot to add in my earlier post.
> The difference is that with a 225/45 tyre, the hight of the tyre is 45% of
> 225 1.25mm
> Using the same calculation makes the hight of the 255/45 tyre, 114.75mm,
> so the overall diameter is greater.

Mike,
Doesn't the 45 refer to the ratio of "width of tire" to "bead  to
tread"
height?
This is based on a cross section of the tire.  Thus, higher numbers result
in wider tires.
Squat.

>
> > Another question is just how much would the ride improve? Has anyone got
> a
>> particular recommendation? Would it be possible to use 225/50R17 (65.68cm
>> diameter) within the arch of the E46?
>
> Dunno.
> Mike.
>

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