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Odd Size Tyres For My New 18" Rims?

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I am going to buy some new 18" ASA W21 Rims. The sizes available in staggered are 8J +et35 and 9J +et38.

The problem is the seller is telling me I need 255/35/18 on the rear, as it wont fit a 255/40/18. (fitting a 225/40/18 on the front).

With a tyre calculator comparing to my current OEM 17" I have worked out that the front will be 0.46% larger circumference which is nominal really, but with the tyre they recommend for the rear will be -2.54% smaller circumference, whereas the 40 profile is 1.36% bigger.

They said I can have the 40 at my own risk as the 40 is 8.5J oem fit only, but why? what is the problem, with the offset and tyre size is there a chance that it will hit the arch? The offset increases the distance from the hub by 18mm on the rear and moves the wheel outwards by 10mm on the front.

I am also lowering the car onto Eibach euro spec (25-30mm drop) springs.

Do you think are they right, those of you that may have changed your wheels...?

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I can't actually answer your question, but can say that our merc has a similar setup on it.

It has 18" wheels, with 8.5J on the front and 9.5J on the rear - and similarly has 40 profile tyres on the front, and 35 profile tyres on the rear.

I'm actually curious now to find out why that is done, these are the OEM wheels and tyres so I'm guessing there's nothing wrong with it, but I did also think that it must mean the rear wheels are actually smaller than the fronts in diameter.......

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