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      09-02-2011, 06:49 AM   #7
kn1
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Drives: F07 - 530d GT
Join Date: Aug 2011
Location: Derbyshire, UK

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Quote:
Originally Posted by Holmax View Post
My merc book recommends 255 on every wheel with winter tyres (normal summer tyres are 285x30 rear and 245x35 front) but as winter tyres were like rocking horse dodahs last year I had to get 265's right round. So I can't imagine them not fitting ok.
It's important to consider the tyre's width and section profile together as these determine the overall diameter and therefore the rolling cricumference (i.e. the distance travelled by one revolution of the wheel).

The section profile is not a fixed unit but is actually a % of the tyre width so in terms of the sizes you give for the merc, a 285/30 means the sidewall is 30% of the 285mm width = 85.5mm; and 245/35 means the sidewall is 35% of 245mm = 85.75mm. So even though they're different widths and ratios front and back they have the same height of sidewall and thus the same circumference.

The 255 winter tyres recommended on your merc I assume would have been 35 series (89mm sidewall height), but if you fitted 265/35 the sidewall height would be 92.75mm, and your starting to increase above 5% of teh original 85mm - which starts affecting your gearing and speedo accuracy (if your overall tyre/wheel is bigger in circumference you're going faster than the car thinks you are!)

If the 265 were 30 series then the height is 79.5mm and your starting to go too far the other way.

Hope this all makes sense.

What profile are the 265's you have and what size are the original 19" summer tyres?
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