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What offset?

2K views 8 replies 3 participants last post by  fastbee 
#1 ·
Hi guys...I'm looking for a new set of wheels/tires. I've found several places to buy from, but I'd like to know what offset I need. I'm looking into getting 18" rims with 275/70/18 tires. I currently have factory 20" wheels and tires. I'd like the new wheel/tire package to fit about the same, but if they stick out just a little I'd be OK with that. Most of the offsets I've found are -12MM, +18MM and +35MM. I'm guessing -12 would be about flush with wheel well, +18 would be a little outside and +35 would be alot outside. Please let me know if any of y'all have and ideas.
 
#4 ·
DC, offset is from centerline so width doesn't matter. It's backspace that you have to consider width with.

I'm guessing you have an '08+ if you have stock 20s? On '05+ trucks the stock offset is +40mm. I wouldn't go any lower than +20mm, and that will move the centerline out about 3/4". IMO, 9" is a little too wide for a 275 tire, consider going to a 285.
 
#5 ·
Keeping a known BackSpace the same the offset changes based on width.....like on 99-04s you want 4.5" Back Space to keep off the leaf springs. That could mean you want an offset of -12mm, -19mm, -25mm or -51mm depending on the width that you want, but they are all equal to 4.5" Back Space.



For a 9" wide wheel a +12mm offset is 5.5" BackSpace, a +19mm offset is 5.75" BackSpace and +25mm is 6.0" BackSpace.
 
#6 ·
like on 99-04s you want 4.5" Back Space to keep off the leaf springs.
I'll have to disagree with you on that. The backspace gives you an idea of where the back sidewall is, but it is still dependent on tread width and wheel width. A narrow tire on a wider wheel will be further from the springs than a wide tire on a narrower wheel, given the same backspace. The easiest way to do it is look at offset, which tells you the position of the center of the wheel and the center of the tread, then given the tread width you can tell how fare the tire will stick out and how close to the springs it will be at full lock.

An example, you say '99-'04 need 4.5" backspace to keep off the leaf springs, however I had 35x12.50 MTs on a 18x10 wheel with 5" backspace and it was still 1" from the springs at full lock on my '03. Those were -10mm offset, so given the same width wheel, I could drop to a 0mm offset and the tires would be tucked under the fenders better and still be 1/2" from the springs at full lock.

I just find it much easier and clear to use offset and tire width. You can use backspace, but then you need the wheel width as well, in addition to the tread width and a little guess work to determine exactly where the tire is going to be.
 
#7 ·
Offset gives you the location of the wheel mounting surface of a wheel in relation to the true center of the wheel. But without knowing width you can't recommend an offset.

For example I run a -51mm offset on my truck which is a 2001..works great, for my truck with my wheels.. But, I can't say run a -51mm offset on 99-04s unless I specify what wheel width I'm referring to.
-51mm offset on a 12" wide is 4.5" BackSpace.
-51mm offset on a 10" wide is 3.5" BackSpace.
-51mm offset on a 9" wide is 3.0" BackSpace.


At the end of the day, offset and backspace are both telling you the location of the mounting surface of the wheel......they just go about it in a different way and you can use one in combination with wheel width to tell you the other.





Back to this thread however and the original poster's question in regards to the different offsets offered.
You actually have it reversed. With a 9" wide wheel:
+35mm offset is going to sit the furthest inward (in excess of 6.25" BackSpace).
+18mm offset will stick slightly out with the increased wheel width (5.75" BackSpace).
-12mm offset will stick out the most (4.5" BackSpace).
 
#9 ·
Back to this thread however and the original poster's question in regards to the different offsets offered.
You actually have it reversed. With a 9" wide wheel:
+35mm offset is going to sit the furthest inward (in excess of 6.25" BackSpace).
+18mm offset will stick slightly out with the increased wheel width (5.75" BackSpace).
-12mm offset will stick out the most (4.5" BackSpace).
I figured that out after my first post...I found a pretty good calculator that shows the realtionship with offset and such.

Cartmanea, I did some checking with local tire shops and they made the same recommendation as you...285 minimum. BTW, I have an '05, need to get my sig created so there is no confusion!

I appreciate the advice guys, now if I could just get my 20s sold and decide on which wheels I want to buy!
 
#8 ·
Yeah, they will give you the same info, I just find offset easier. What you're really concerned about is the location of the tire, which is directly related to offset OR backspace and width. When comparing wheels, its easier to look at one number for tire position as opposed to two and doing some math. I will say you're wrong about not being able to recommend an offset without knowing width though. When you're determining your tire clearance, that is directly related to offset, width doesn't matter because the tread is what causes interference and tread width doesn't change with wheel width. Sidewall width will change slightly, but that only affects functionality for running low pressures and how it looks, you don't get sidewall rub. You can get the same info by backspace and width, but using offset is more direct.
 
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