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farrell said:
Anyone aware of any issues fitting 245/45/17 michelin pilots onto a 9 inch wide rear ?
Its not a huge difference in circumferance & rolling radius.
Maybe using this profile could make the rear feel different.

Any comments please.

why you doing this anyway mate?


2,415 Posts
Factory fit for the 9" is 255/40/17.

Changing to 245/45/17 from 255/40/17 would result in;

Geometry Calculations:
change in ride height 8.3 mm
inwards add'l width req'd -5.0 mm
outwards add'l width req'd -5.0 mm
add'l total tire height 16.5 mm
add'l vert clearance req'd 8.3 mm

additional rolling diameter 51.8 mm
change in drive ratio 2.6 %

A better bet would be 245/40/17, the 40 profile rather than 45, it`s a closer match.

You`d get a slight reduction in understeer at the expense of less traction at the rear.

You might want to consider leaving the rears at 255/40/17 and changing the fronts from 235/45/17 to 245/40/17. This will tramline more and put more stress on the steering, but lessen understeer.

I run 265/40/17 and 245/40/17 in Pirelli. Overall traction is wonderful, tramlining not so good, plus it hurts the wallet.

Next on the list is the 20mm rear antiroll (sway) bar. This is supposed to make the car far more neutral understeer-wise when used with the big 9"J rear wheels in 17".

Hope this helps.


Moderator Emeritus
4,273 Posts
Discussion Starter #4
Hi Ivan
Like you, I did the calcs this morning. The figures you & I have are theortical nominals & the tyre guys can be very cute on true sizes of their tyres

Your comments on understeer & oversteer are the ones I was hoping to get.

An added complication is that the tyre manufacturers work on ETRTO figures that allow tyres of the same size from different manufacturers to vary hugely in diameter & rolling circumference & can be as much as 3 to 5 %
difference either side of nominal figures. The tyre manufacuturers do supply the O/E vehicle manufacturers the actual true figures as opposed to theoretical max / mins.

For me this is potentially a huge cost saving so am willing to give it a go as long as someone does not throw a big spanner in the works..

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