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From digest.v7.n519 Fri Oct 31 06:39:36 1997
Date: Fri, 31 Oct 1997 00:40:43 -0500
Subject: Wheel Offset

In v7 #515 somebody asks about wheel offset. The poster has a 6 series and he wants to put larger, wider wheels on it while keeping the stock look. Figuring wheel offsets gets real confusing real quick.

First you should find out what your current wheel offset is. At least on my BMW wheels, you can find this real easy. Just take one of your wheels off and look on the inside. You should see a BMW part number and a number that begins with "et". You should also be able to tell how wide your rims are. Hopefully your rim width is stamped in the rim.

I am going to guess that you probably have about 25 mm offset rims and your rims are 6.5 inches wide. So if you want to put 8.5 inch rims on and not have the tire stick out any more than stock, then you have got to pull the rim inside the wheel well. The rim is 2 inches wider so that means it is going to stick out of the wheel well 1 inch more than before, keeping the same offset.

But if you don't want the tire to stick out more, you have got to increase the offset by 1 inch or 25.4 mm. This will give you a new offset of about 50 mm. This was a long winded way of describing how to change rim widths and keep the outside rim edge even with the original wheel. The mathematical formula is:

[(new rim width in inches - old rim width in inches) / 2 * 25.4] + old offset in millimeters = new offset in millimeters

The next question is do you really want to do this? Do you have 2+ inches of clearance on the inside of the tire? Will the tire rub on the struts, inside wheel well, muffler, etc? I doubt if you will be able to completely tuck the wider wheels inside the car without something rubbing.

I also doubt if you will have much chose of offsets for you new rims. Unless you are buying very expensive, custom made 3 piece rims, I think most aftermarket rims are going to have about 35 mm of offset. This should work OK for you. The tire will stick out of the wheel well a little bit more than stock but you won't have a problem with rubbing on the inside.

The key to remember is: Increase the offset to keep more of the wheel tucked inside the car. Good luck.

Lowell Seaton
'95 M3

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