biggest tire size

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blmopar1

blmopar1

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I have a 68 fastback.Without relocating springs what is the largest size 14'or 15" tire that can go on the back . I Wanted to put a 275 tire with an offset rim. Thanks:happy6:
 
I don't think you can get 275s under it without any mods. How do you want the car to sit? If you want it to sit stock I think you would have less of a chance.
 
When I asked the question at Stockton Wheel, the answer I got was a 235/60-15 on a 7" rim with stock offset. There are guys on the site who swear you can get a 245/60-15 on the back. I tried a 255/60-15 at one point. It fit in the wheelwell but was only about a ¼" off of the rear spring. There are some other threads @ FABO that have discussed this topic before.
 
These are 255/60-15's on a 15x8 Ralley wheel with 4.5" of back spacing.

IMG_0810.jpg


I have about 1/2" clearance to the spring but well over an inch to the wheel well lip. With a rim with less back spacing a 275/60-15 tire would fit.
 
Interesting... I had seen that 4.25" of back spacing was the max. Are your springs and hangers in the stock location? If so it opens up more nice options for people.
 
Springs and hangers are in the stock location. I would think a 4.25" back space rim would allow a 275/60-15 to fit.

I have seen lots of A-body Barracudas, Dusters and Dart Sports with 275/60-15 and even 295/50-15's on the back. These versions of the A-body have the biggest wheel wells.
 
1968FormulaS340 said:
Interesting... I had seen that 4.25" of back spacing was the max. Are your springs and hangers in the stock location? If so it opens up more nice options for people.
Click to expand...

An 8" wheel with 4.5" backspacing would probably allow for a wider tire than a 7" wheel with 4.25" backspacing. The same size tire on the 8" wheel would actually move away from the spring by a bit more than .5", depending on variations in the cross section of the tire due to the different wheel widths. You're moving the inside edge of the wheel .25" closer to the spring but the center of the wheel, as well as the center of the tire, is moving away from the spring by .75". I think too many people get hung on backspacing measurements. It's the offset that determines where the tire sits in the wheelwell.
 
You can fit a 275/60/15 on 15x8s in there with no problem if ... you use the offset shackle kit. I installed the kit and then had the offset on the 15x8s adjusted 3/4" and it fits perfectly. It is wise to take measurements before doing anything since we all know that Ma Mopar's idea of centering her rear axle housings was not always that centered. ;)
 
I measured the backspacing for my wheels. The front wheels 18" X 8" are 5 3/8" backspacing.... The rear wheels 18" X 9.5 are 6 1/4" backspacing.... I moved the rear springs in with a .80" off set hanger to make room for the wide wheels and tires, also I centred the rear axle housing to the car for proper fitment. The wheels and tires are the widest that can fit in the stock well. I have just 3/16" clearance betweent the tire side wall to the quarter panel lip and 3/16" from the side wall to the leaf spring. It works well, only because of the low profile design of the tire. You will need a strong side wall or it will rub. Hop this helps you. Let me know what you do.

101_0133.jpg
 
'64 Cuda said:
An 8" wheel with 4.5" backspacing would probably allow for a wider tire than a 7" wheel with 4.25" backspacing. The same size tire on the 8" wheel would actually move away from the spring by a bit more than .5", depending on variations in the cross section of the tire due to the different wheel widths. You're moving the inside edge of the wheel .25" closer to the spring but the center of the wheel, as well as the center of the tire, is moving away from the spring by .75". I think too many people get hung on backspacing measurements. It's the offset that determines where the tire sits in the wheelwell.
Click to expand...


Whether you use offset or back spacing you get to the same answer. It just more common to use back spacing when you are talking about muscle cars. The term offset became common with FWD cars that have very large back spacing for the rim width. Zero off set or back spacing that is 1/2 the rim width centers the rim/tire on the plain of the wheel mounting surface on the axle. In your example the 8" rim with 4.5" of back space is .25" closer to the spring than the 7" rim with 4.25" of back space.

Second generation Barracudas, Dusters and Demon/Dart Sports have relatively large wheel wells and its the location of the leaf springs that limits the tire size before the actual wheel well.

In my case above I have a 8" wide rim with 4.5" of back spacing, that is .5" of positive offset. With a 255/60 tire mounted the tire comes within 1/2" of touching the leaf springs. By going to a wheel with 4" of back space, zero offset, I move the entire rim/tire outward 1/2" away from the spring which would give me 1" of clearance to the rim and about 1" clearance to the wheel well lip. A 275/60 tire generically has a cross section that is .8" wider than the 255/60 or .4" per side or .6" of clearance on each side.
 
I have the same as DGC333, 255/60 15's on 4.5" backspace 15x8's.

I was thinking of trying 275's next......
 
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