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Old 07-23-2010, 08:09 PM
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How to Calculate Wheel Offset.

I need to figure out what size wheel I need to fit in the rear wheel well from the axle face to the outside wheel well opening. If I know the that distance, how do I calculate the offset? What does negative & positive offset mean?


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Old 07-23-2010, 08:56 PM
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Zero offset is the wheel mounting face in the center of the rim. Positive offset is wheel center closer to the inside rim edge making the wheel look deep. Negative offset is wheel center closer to the outside rim edge, wheel looks "flat faced" like most of todays cars are.

Most wheel companies use "backspacing" for measurement, this is he distance from the wheel mounting surface to the inboard(backside) rim edge.

Example 15 x 8" wheel with 3.5" backspace leaves 4.5" from mounting face to outside edge of rim. Wheel has a "deep dish" look to it. In offset terms, this would be a 1/2" positive offset, center of wheel is offset 1/2" to the inboard side of true center of the wheel.

EDIT: It has been brought to my attention(thanks) that I have positive and negative backwards , so swap them in the above description. Backspace info is correct.

Last edited by ericnova72; 07-24-2010 at 12:45 AM. Reason: Wrong info
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Old 07-23-2010, 11:01 PM
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I ran into that on my 88 formula. The front rims had more negative offset to clear the tie rod ends on the front end. Tried to switch the front with the rears once because the rear's with pos. offset looked like the rims were on wrong with the deeper looking rims on the front. That sucked because now, I had to dismount the tires for rotation because of the directional tires, and the wheel balance weights hit the tie rod's. There was only 3/4 in difference between the front and rear.
Take this into consideration when you buy new rims because if you get different off set's you might run into this problem.

Last edited by sqzbox; 07-24-2010 at 05:12 AM.
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Old 07-24-2010, 12:20 AM
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Chart here>> http://members.cox.net/completevalue...at.html#OFFSET
http://members.cox.net/completevalue/wheelpat.html

Eric, by this chart your negative/positive is backwards.

Last edited by SSedan64; 07-24-2010 at 12:25 AM.
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Old 07-24-2010, 12:41 AM
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Quote:
Originally Posted by SSedan64
Chart here>> http://members.cox.net/completevalue...at.html#OFFSET
http://members.cox.net/completevalue/wheelpat.html

Eric, by this chart your negative/positive is backwards.
Thanks for catching that , I was worried after I thought about it but for the life of me couldn't truly remember which way it was. I worked right out of High School at a place that made Sprint and Modified wheels, but we always used backspace measurements, like most wheel manufacutere do. Never used offset, it is easier to get confused using it.

I'll go back and edit, thanks again.
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Old 07-24-2010, 05:14 PM
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Thank's Guys... That is a nice chart and a real good explanation. I set this car up for a certain off-set and forgot what it was. The wheels came up missing ...

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Old 07-26-2010, 11:19 AM
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Here is a post from another member. A very nice homemade tool to measure fitment of tires/offset.

Basic Tire/Rim Selection & Fitment Help
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Old 07-27-2010, 12:40 PM
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Thank's for the link to the homemade tool. I made one this weekend and it works great. Really gives me a feel for tire size & offset.

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