So much BS...15 x 8

Back spacing that is...
in the pics, 255/60/15 x 7 on a 71, 8.5 in a 67GS. (auto algebra 101)
now the 71 rearend is 1/2" wider on each end then the stock 8.2
i'm going to put 8 x 15 in the rear but looking for confirmation on the rear BS.
from whats shown my thought are 5.25 BS on 15 x 8
i believe the last time I measured the 15 x 7 they were 4 1/4" BS.
Is the 5 1/4" BS on 8's to much for the rim ?
also is my math right ?
EDIT: Is adding 1" more BS enough, or am I just adding the 1" to the rear ? should add 1.5"
that would be 5 3/4" BS ?

 

if i was using the same 7" rim it would be easier. adding another inch and having 5 3/4" BS is the question of is it to much BS for that size rim

 

My 66's, with the 15 x 7 Buick mag, need another 1/2" backspace to center the tire. Wheels have 4- 3/8" backspace, so 4- 7/8" would be ideal. With the wider rear you're using, add 1/2" for a total of 5- 3/8" backspace.
The repo wheels have different backspace, so check yours. 4" I think.
Backspace with a wider tire, like a 275 is critical. With a 255 you'll have a little room to play with. If you can find a 5-3/4" backspace 15x8, it should fit. Worst case is you may need a 1/4" wheel spacer to move the tire out a hair. If you use a 5" backspace wheel, there isn't much you can do if it rubs your fender lip except to bend the lip up for clearance.
There are wheel fitment tools out there if you want to double check. Percy's wheel fitment tool is one.

 

Thanks Walt, i'm going to have the rear rims made to spec, i'm pretty confident on the 5 3/4 BS. From what I've read, the tires won't bulge out as much with an 8 inch rim as well. that's what I was looking for, a little confirmation before moving on it. what do you run up front ?

 

235/60-15's up front on the 15x7 Buick mags. They fit fine. 255's on the rear now, I used to have 275's. The 12 bolt rear tucks them in 1/2" on each side.

5- 3/4" backspace may move your tires in a bit more (3/8") than what I show here. With 255's you should be fine. If you go to a 275, it may need some spacers to get it perfect. It's also difficult to compare what's on my car vs what's going on your car, especially when we're talking of such a small amount.


Here are some pics of the 275's. Note I bent the fender lip and trimmed the ww molding for more clearance. I did this in the 80's when original moldings weren't worth so much! Clearance on either side is about a finger-tip, about 5/8"- 3/4".



And a pic with the 255's on the rear

 

that looks great. perfect stance for the car. the tires fit nicely too. I used the cc501 moog cargo springs in the rear. stock fronts. I'll try some 235's up front today. If I can get my 67 to sit that nice i'm happy.
I think I'll stick with the 255/60/15 x 8 rear w/5 3/4" BS & 235/60/15 x 7 front. do you have stock front springs?

 

Been a long time since I installed those front springs! I think they're Moog GTO spec. Rears are Special wagon springs I cut down for a lower height. Moog 6197 are a good choice for 64-66 rear, about the same height as my blue 66.

 

5.5” backspace fits good. When you go deeper,like 5.75”,with a 15” rim,the first object you will hit will be the shock mount,where the nut is. 5.5” BS clears that,but is close. 5.75” will be closer,but should clear. 6” will hit,but you can get creative with the mount,or the style of shock bolt. The only other issue you might have is if you are running 3” tailpipes. Depending on how they are routed,and how much they are tucked,they might rub on the tire. I had to “ovalize” that section of pipe,with a block of wood and soft mallet.
One last question: are you running rear drums or discs? No matter what rear disc kit you run,the rotor hat is thicker than a brake drum,so that will push your wheel outboard. It depends on what rotors you have ,but it can range from 3/16” to 1/2” additional width,so more backspace would be needed to compensate.

 

Hi Brian,
5.75" is where I believe I need to be. i want to move the front of the rim in 3/8" from the rear QP lip. right now it's right on it. it's a 255/60/15 on a 7" rim so it's the bulge in the sidewall of the tire that will rub. the tread clears.(pic in OP) I am running 3" pipe however it is not installed yet. finally I have stock rear drum.
I'll measure my stock 15 x 7 and see how much space is available between the rim and shock mount.

 

https://www.willtheyfit.com/

Try this website. Gives you a good visual.

 

The sidewall of my 325/50/15 tires were barely clearing my Torque Tech 3” tails,and there were some light run marks from the tire. It was the area just on the side of the tread/sidewall. That is when I massaged the pipe very slightly. Tappy Tap Tap.

 

What Brian said: "5.5” backspace fits good. When you go deeper,like 5.75”,with a 15” rim,the first object you will hit will be the shock mount,where the nut is. 5.5” BS clears that,but is close. 5.75” will be closer,but should clear. 6” will hit,but you can get creative with the mount,or the style of shock bolt."


The shock bolt in the picture is a Monroe AK32 part number. You can find them on-line.

 

5.5 wont do it, the 1971 8.5 rear end add 1/2" on each side. I need to absorb that as well

 

The figures I gave you would be with the 71-72 8.5” 10-bolt rearend. 5.5” will clear,5.75” will be real close,6” will hit the shock mount.

 

the shock mount is definitely in the way. why can't i move the shock mount up 1 inch? drill one hole and it's clear

 

I think that is going to require you to change shocks to a shorter stoke, remember that shock keeps the rear from falling off.

 

Good point I wouldn’t want the springs falling out. Ok well thats an option

 

That was a quick fix back in the day,when the air shocks would rip the hole off the end of the bracket. They would drill a new hole higher up on the bracket. It “will” work,but yes,it limits the stroke of the shock,and also puts the shock at even more of an angle that it is already. You could fab up a bracket,to mount to the existing bracket,to move the shock inboard.