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: