E65 wheels on E31

When some good condition E65 18″ wheels came up locally with nearly new tires it seemed a good bargain, and would hopefully cure some rear vibration where I suspected a problem with one of the rear wheels.  They are ‘style 94′ and came as standard on many E65/E66 cars.

I knew plenty of other E31 owners run 18″ wheels, and I know the center bore (72.6mm) is identical between E31 and E65, as is the bolt pattern (PCD 5×120) so what could go wrong?  Well, I completely forgot about the tire aspect ratio!  The new wheels & tires simply did not fit in the space available under the mount for the spring.  Where did I go wrong?

The old tires are 235 45 17 – that means 235mm width, 17″ inch wheel and 45% sidewall aspect ratio.  In other words the side of the tire is 45% the width.  So if we convert that 17″ to mm (431.8) and multiply the sidewall by 2 for top and bottom we end up with:

(235 * 0.45) * 2 + 431.8 = 643.3

The new wheels are 245 50 18 – so if we convert 18″ to mm (457.2) we get:

(245 * 0.5) * 2 + 457.2 = 702.2

So that’s a difference of almost 59mm – no wonder it didn’t fit!  In fact the clearance available is more like 10mm.  It’s worth noting even if it had fit running a vastly different size like this would have upset the speedometer as the rolling radius will be different, and could potentially confuse DSC/traction control as it too will find the electronically measured rotation is different from the physical one.

So this meant different tires, a good online calculator for working out sizes is http://www.rimsntires.com/specs.jsp

So by switching to 235 40 18 tires the overall height is now within a millimetre of the original and all is fine.  It’s also worth mentioning the ‘offset’ of the wheel – this is the distance from the hub to the mounting surface of the wheel.  You should always be able to find this stamped on the inside of the rim – for the E65 wheels this is 24mm – the E31 spec is 15mm-25mm so it’s just inside!  If the offset is greater than 25mm then the wheel would not clear the strut body, but a spacer could be fitted to compensate for this.

Old and new pictures: