How big of rear tires can i get?

Been reading a lot of this in various posts. There's been some good info. For my car, and probably most, it's going to be the frame or body that's limiting. Backspace is critical. I drew up what the left side of my 68GS400 looks like with a 12 bolt in it right now. My concern is that a CAD drawing of a car on jacks doesn't take into account the slight movements of a body or tire under load or turning. How much space do I need to avoid rubbing? I have both 255/60R15 and 275/60R15 shown below. I hope it's correct!

I'm not concerned about the shock, which fits inside the rim on wider rims. I anticipate rubbing of the 275s on the frame with 15x7s... Only 0.6" of clearance and 5" Backspace. It seems like less backspacing would be better for this car and 275s.

NOTE: dimensions have been updated since original post.






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This site shows 10.2 on a 255.

 

Another site shows this:




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Thanks Larry. That will shave off about 1/8" roughly on each side. That would leave me roughly 0.7" to the frame with 275s, 8" rim and 5" BS. So how much distance should a person realistically expect to keep from the frame and body. I don't want to be cringing any time i turn or am 5 pounds low on tire pressure.

Seems like 4.5" BS is more standard for rally wheels. That should help me; correct?

I'll update the drawing in a day or two. Maybe it will be helpful to others to have it. A person could just put in the dimensions for their specific vehicle.



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What are you doing for brakes ?

I had more trouble getting the wheel to fit around the disk brakes.

What wheels are you going to use ?

 

Just in the planning stage. Keeping stock drums in the back. I'll just use my big front bumper to stop. Lol.

I like the classic rally wheel look as well.

If I go with 4.5" bs, i should have about 1" to the fender. How big of a deal to go with the 255 instead of the 275s?


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Eight or ten years ago there was Crystal Blue '69 GS 'vert at the 'Nats that was running Wheel Vintiques (if I remember correctly) 15"x8"s and 255/60s all around. It looked mean and they fit well.
Patrick

 

I'll recheck the distance from drum to fender in multiple locations later. It surprised me as well, as I have read 5" BS for a 68-69 in some posts as well.

Is a 12 bolt Chevy differential the same length as the original Buick differential?

 

I updated the tire widths in the original post.

The other side (passenger side)is about 1/2" closer to the fender. 1/2" further from the frame.

So if I subtract 1/2" to make it 4.5" bs and subtract another 1/2" for the other side being 1/2" closer to the fender, that gives me about 1/2" clearance to the fender on the passenger side. With tire movement and body movement, that doesn't leave a whole lot of room for a 275.

If I leave it with 5" bs, then the limiting location is the left side frame which is 0.7" of clearance. Since a frame shouldn't move/flex, I'm guessing it's probably better to be closer to a frame than a body, if given a choice.

I can see how 5" bs is preferred for the 68-69. Seem reasonable?

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I only know of one in town. I'm not sure what he has for wheels; I'll have to ask.

Be careful what you volunteer for, I may drive hundreds of miles to do it! Just kidding.

 

I'll try yo get out in the garage tonite and take some pics - my GS has a 12 bolt in it and the wagon's got the stock 8.2 (for now). Both are wearing 275/60/15 cooper cobra's. One of them has the 15x8" rims with the 5.125" backspacing I had them custom made to. The other is wearing stock 15x7"s. Cant remember the frame ever being a problem - its either the lip in the center of the wheel well that sticks out past the moulding AND bumps out in the middle, or, if you reach up about 5" inside the inside of the outer wheel well, there's a stamped ridge that runs horizontally to provide rigidity to the panel.

You can re-center the body on the chassis but it involves undoing almost all of the body-to-frame bolts then lifting it and re-centering .. but in most cases will cause a serious headache when you spin the first cage nut. When you spin the second one (usually a hidden one ..) you really regret starting the process and the headache turns into a migrane

 

I don't need anymore headaches in my life!

I may just go with 255/60r15 to be safe.

 

I just put on a pair of 325/50 Nitto NT 555R Drag Radials and they fit just right. :TU:
Wheels are Centerline Auto-Drag 15x8.5 with 4.875 BS
Car is a 72 Skylark with a 68 or 69 12 bolt rear-end.

Tires are great so far. Very little/brief tire spin at full throttle launches on the street. :Brow:
Previous tires were 295/50 BFGs...they just spun and spun and spun. :spank:

 

this is the left side of the GS with the 12 bolt rear, 275/60/15 tires and I believe the sstock 15x7" rims judging by how much room to the frame I have. looks to be a good solid 2"



I Did not get a pic of the wheelwell opening to wheel .. I can take that tonight but I cut my wheelwells and moldings many years ago (stupidity + slicks).

This is the SportWagon, stock 8.2" rear, same 275/60/15 tires on the 15x8" rims with 5.125" BS. Looks like its just over an inch



Here's the wheel well opening to tire - 3/4". I also modified these openings but a bit more intelligently by taking the lip back to the trim with a sanding flap disc in a die grinder - I initially marked it with a yellow grease pencil, took the trim off, then slowwwwly worked it letting the heat dissipate.

 

The 70s have a bigger wheel well. That's wheel well envy for us 68 guys.