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Author Topic: Tyre sizes and rolling radius  (Read 249 times)

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pauls

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Tyre sizes and rolling radius
« on: 06 May 2018, 16:24:36 »

I picked up a bmw which is running on 235 45 19 on the front and 255 40 19 on the back. It needs new tyres all round but i cannnot find 4 of the same. Apart from the cheap rubbish.
 Is they any reason why i cant run 235 45 19 all round. I think the alloy is a bigger width on the back.
Also is the running diameter different with the setup it has now.
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Doctor Gollum

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Re: Tyre sizes and rolling radius
« Reply #1 on: 06 May 2018, 16:26:23 »

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pauls

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Re: Tyre sizes and rolling radius
« Reply #2 on: 06 May 2018, 17:00:21 »

Ok so using that..
255 40 19 sidewall 4.0" radius 13.5"  diameter 27.0" circumference 84.9"

235 45 19 sidewall 4.2" radius 13.7"  diameter 27.3" circumference 85.8"

These are my tyre figures but how does that help with my question. Does it mean i will be fine running the same 235s all round even though i think my back wheels are 8.5 not 8 j.

Sorry if questions are stupid but price of tyres are not cheap so dont want to cock it up
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dave the builder

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Re: Tyre sizes and rolling radius
« Reply #3 on: 06 May 2018, 17:05:02 »

I use this calculator
clicky

try the sizes you can get and compare  :y
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Doctor Gollum

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Re: Tyre sizes and rolling radius
« Reply #4 on: 06 May 2018, 17:11:43 »

Are they standard wheels? If so, suitable alternative sizes should be listed in the handbook  :-\
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pauls

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Re: Tyre sizes and rolling radius
« Reply #5 on: 06 May 2018, 18:18:03 »

Yep wheel are oem. No other size tyres. Only other option is buy 4 new wheels 18"
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Nick W

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Re: Tyre sizes and rolling radius
« Reply #6 on: 06 May 2018, 18:55:16 »

Ok so using that..
255 40 19 sidewall 4.0" radius 13.5"  diameter 27.0" circumference 84.9"

235 45 19 sidewall 4.2" radius 13.7"  diameter 27.3" circumference 85.8"

These are my tyre figures but how does that help with my question. Does it mean i will be fine running the same 235s all round even though i think my back wheels are 8.5 not 8 j.

Sorry if questions are stupid but price of tyres are not cheap so dont want to cock it up


the ACTUAL circumference will change depending on the tyre; there isn't a proper standard on where the the physical tread width is measured. A difference of 0.9mm on the circumference is unlikely to make a difference, but if clearance is tight the bigger tyre might not actually fit the car.
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