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Discussion Starter · #1 ·
alright so i have an 1988 mustang lx. im doing a five lug swap in a couple of days, im using sn95 spindles up front and ranger axles in the back so im retaining the stock axle lenght. i want to run deep dish wheels in the rear.(bullits or fr500) what kind of offset or backspace do i need?

americanmuscle.com has the following wheel available

18x9: BS(backspace) 6.3 OS(offset) +34
BS 5.94 OS+24

18x10: BS 7.2 OS +45
BS 6.3 OS +22

17x10.5: BS 6.8" OS +28mm



if i go with the 18s i would have to run a 18x9 in the front with a 245 tire
if i go with the 17s i can go with a 17x8 with a 245 tire

Please help i don't know what to do, all help will be apppreciated. thanks
 

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I just went through all of this and came out with a perfect fit. It is a very complicated measurement process.

My 1989 LX wound up needing a 5.5 inch backspacing for an 11 inch section width tire. This put the fattest part of the tire about 3/8th inch from the closest obstruction which are the quad shocks and the exhaust pipes. If you use MORE backspacing, it will move the tire closer to the inner wheel well.

The factory backspacing on my wheels was about 4.8 inches. I moved the tire inward about 3/4 of an inch by using a 5.5 inch back space. 4.8 - 5.5 = -0.7 inches If you use a 6.5 inch backspacing the tires inner edge will move about 4.8- 6.5 = -1.7 inches. Look at where your tires are now. Could you stand moving the tires inner edges in almost 2 inches??? I bet not.

The outer edge location is set by the tire section width and the rim width.

If you make the tires 2 inches wider, and if your stock tires are perfectly centered in the wheel wells on stock rims (they won't be), you would want to INCREASE back spacing by one inch to keep the tire centered.

My factory wheels and tires had about one inch more outer spacing than there was inner spacing. Because I had more room going out than in, I did not move the wheel in the full amount or the tires would have rubbed.

I moved the back spacing up about .6 of an inch so the wider wheels and tires moved the outer edges out more. The inner edges with the wider tires came in about 1 inch, but the outer edges moved out about 1.5 inches.

I really think you will get into trouble with a 6.5 inch wheel backspace unless the tires have a section width close to the rim width. Remember the factory is just under 5" back spacing. You can measure your factory backspace easy with a ruler and straight edge.

Lay the straight edge across the inner wheel bead at the tallest metal edge. Measure the distance from that straight edge surface to where the wheel sits on the drum. That is the back space. As you make it more and more, the inner wheel edge moves in more and more.

By some careful measurements and looking at the tire charts, I was able to get within about 3/8 of an inch from the fender lip and from the inner fender area! I had to use an 8 inch wide rim with 5.5 inch backspacing. My rear tires are 10.50 x 27 - 15's. When I get the rear end centered the tires will clear the inner obstructions by 3/8th inch, and the outer roller fender lips by 3/8th inch.

Tom
 

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Discussion Starter · #3 ·
whoa, that is a lot of information, i appreciate it but, i don't understang very deeply how this works. are you trying to say that i should run a backspace of 5.5??? im also removing the quad shocks and im also rolling the lips
 

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Discussion Starter · #4 ·
by the way the tire i'd like to use has a width of 11.5 i believe, thanks for the information and the hehlp:bigthumbsup
 

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I can't tell you what you should use, because if something rubs I don't want to be responsible.

I can tell you how it works.

1.) The backspace sets the distance the rim moves in or out on the INSIDE near the shocks.

2.) A BIGGER backspace moves the tire and rim closer to the shocks and inner wheel well wall.

3.) A smaller backspace moves the wheel and tire OUT towards the fender lip.

4.) The stock backspace on my Pony wheels measures about 4.8 inches.

Look at how your stock wheels sit now on the inside, and where the inside of your tires are at. Decide how far in you can move them. If you can move them in one inch, get a 5-3/4 inch backspace. If you can move them in 3/4 inch, get a 5-1/2 inch backspace.

Personally I think a six inch backspace is pushing it in too far unless the tires have a small section width compared to rim width. If the tires are real low profile it will probably be OK, but if the section width is close to what mine are (11 inches section width) they will be pretty close to the inner fender or even touching with a 6 inch backspacing.

Tom
 

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Discussion Starter · #6 ·
thanks for the help, i understand how it works now better:bigthumbsup
 
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