In general people seem to be confused by offset and backspacing, but this stuff is pretty simple.
No algebra, just addition of positive and negative numbers and division.

Backspacing is really just the measurement one would get by laying the rim on the ground, (ugly side down), and measuring from the ground up to the mounting pad's surface.

The centerline is the total wheel width including the lips or flanges divided by two. Currently most wheel manufactures and dealers add 1 inch to the nominal/advertised wheel width to account for the lips. So centerline of an 8" rim is 4.5" not 4".

Offset is just the measurement of the wheel's centerline to the mounting pad. This is usually stated in millimeters, (mm), and can be a positive or negative value depending if the mounting pad is closer to the front or back of the rim, (from the centerline).
With a positive offset from the centerline, like +25mm, the mounting pad is closer to the outside/finished surface of the rim. Because of this the wheel is pushed in toward the vehicle before the mounting pad makes contact with the hub and can be bolted on. And the backspacing is greater.
For negative offset the wheel's mounting pad is moved towards the inner surface/unfinished part of the rim, from the centerline. For example -12mm offset moves it inward by about a half an inch. When you go to mount the wheel the mounting pad makes contact with the hub sooner, and therefore the wheel sticks out more. And the distance from the back edge of the rim to the mounting pad is less, which is the backspacing measurement.

To calculate backspacing:
1) Take the nominal advertised rim width, 8" in this case
2) Add an inch for the lips, so 9".
3) Divide by 2, so 4.5" (known as the centerline).
4) Add the offset, +25mm, (+1"), in this case, so you have 5.5" backspacing, which is the same as the OEM.

Now an 8" rim with 0mm of offset would have added 0" at step 4, so the backspacing would be 4.5". Or for a 9" rim, at step 2 you'd add the 1" for lips making 10" of total width, so at step 3 centerline would be 5", and at step 4, if a 0mm offset, then 5" of backspacing. Or if the offset was -12mm (which is about -1/2"), you would add that to the 5" centerline measurement and get 4.5" of backspacing.

Very nice! I was never sure on the mm to inch thing.
I guess I dont have to hassle you for the info anymore.
Thanks

The exact conversion is 25.4 mm to an inch and 12.7 mm to a half an inch. (don't want you loosing any bar bets).

No hassle at all. I helps me to remember. It's a foreign concept for me to visualize that a more positive offset on a rim means the rim is tucked in more, and a smaller backspacing means the rim sticks out more.

Hey Swarren, you should work at a Tire/Rim shop. If you don't already....Good info.

Thanks, that means a lot to me.

I've thought doing that when I retire, (from the silicon mine). hmmm ... I wonder if tire shops hire people in their 70's or 80's ...

Probably not, just open your own store. I'm in the tire equipment business hence the name equipingu. I represent all that fancy tire equipment such as tire balancers, tire changers, brake lathes, aligners and racks. So when you decided to open your business call me . Not that I'm trying to sell you anything.....

Please pm your business contact info. to me. I'll probably won't get the opportunity for starting a business, but who knows ...

These days I'd think one would have to be part of a chain, like an independently owned store, with a famous name association , or a franchise, if they exist in your industry.

Thanks again.

Who needs The Tire Rack with you around??

Very nice writeup and it cleared up a few things for me too!!

David

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great post... thank you.

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Jeremy



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Here is my new wheels and tires.
18x9 with 4.75" backspace. 35x12.5 tire.
It sticks out of the fender well a little bit but at my height it didnt need a pinch weld mod but at full turn and compression it would have hit so I did the pinch weld mod anyways.
Under normal driving they would have been fine.

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