275/60/15 on a Cuda (long)

I decided to upgrade the wheels and tires on my 70 Cuda. Examination of the archives here on Moparts showed that a mixed opinion on whether 275/60/15 would fit well in the stock wheelwells. I wanted to go with 15" rims and the most tire I could fit. I ended up doing something different from what I had seen before and wanted to share what I did in case it ends up providing value to someone else.


Careful measurements of the distance from the brake drum mounting surface to the leaf springs and the wheelwell fender lip showed that the fit with 15X8, 4.5BS and 275/60 would be real close but possible. Of course it was hard to know the exact shape of tire and were the point of least clearance would be. I decided to go with American Racing Torq Thrust IIs.

Once I mounted up the rims and tires on the back I found that I was not completely satisfied with the fitment. The tires did not make contact with anything, but with sufficient suspension compression (perhaps 2") the inner fender lip could make contact with the tire sidewall. Given that my car is mostly street driven, this was a concern to me. In addition, the appearance was also not quite what I was looking for. The tires stuck out a little bit, and were not fully tucked under the wheel wells. I know some run this exact setup and will say that the fit was OK, but to me it was not quite right.

Here, you can see that with enough suspension compression, that there will be contact.

Views of the fitment.

Another view...

The leaf springs on a Cuda are not parallel to the frame rails. With the 4.5" BS wheels in front of the axle there was 1.5" of clearance between the tire sidewall and the spring; behind the axle there was only 3/4" of clearance. Again I turned to the Moparts archives to see if there was clear consensus on how much clearance was needed to be safe. Unfortunately there were mixed opinions on this. In the end I decided that 1/4" of clearance behind the axle after going to 5" BS wheels was not going to be safe enough, again because it was a street car.

Some folks put B-body rears under an E-body to allow for more BS. I may consider that at some point as well, but if I did that I would go with a Dana, and there was no funds to undertake such a project at this time. The other option was to move the rear spring perches on the 8 3/4. That would violate my "nothing but bolt-ons rule", and besides, I was not up to welding, and didn't know anyone I would trust to do it either. I was stumped on how to make this work the way I wanted.

After pondering the dilemma for some time, I finally had an insight that lead to my current solution. There was plenty of leaf spring to tire sidewall clearance in front of the axle, and where I need the additional clearance was solely behind the axle. The wheelwell itself had more than adequate clearance for the 275/60/15. I realized that if I could pivot the leaf spring about the spring perch locating pin, then I might be able to use more backspace without having to cut off and re-weld the spring perches. Based on the stock E-body leaf spring dimensions (22" front segment, 35" rear segment), it appeared that if I moved the front eye of the spring OUT away from the centerline of the car by 1/2", then I would gain 1/4" of additional clearance behind the axle. If I could achieve that then I would have 1/2" total clearance between the spring and tire sidewall.

Moving the front spring segment towards the tire was a bit counterintuitive, but I was hoping that perhaps a front eye hanger from another body style would help. At that point I called the good doctor (DrDiff) to consult on the feasibility of what I was trying to do. He indicated that I could try to use his A-body 1/2" offset front hangers, mounted on the wrong side of the car to change the offset direction, and his E-body 1" offset rear hangers. The thinking was that there would be enough play in the arrangement to use the 1" offset instead of the 3/4" I had calculated. Cass told me that Use of the A-body front hanger required a 5/8" hole, but that was easy to accommodate. The other downside was that the A-body front hanger could not directly provide the stock E-body height; there were two holes for the spring bolt, and one would lower the car by 1/4" and the other would raise it by about 1" (if I remember what he told me correctly). I decided this was close enough to try!

The assemble was straight forward, and I decided to use the upper mounting hole for the eye of the spring. The axle perches easily set down on the locating pins. I could now see that the leaf spring was almost parallel to the centerline of the car. Measurements from the brake drum mounting surface confirmed that all was going to plan. I ordered up a set 5" BS custom offset 15 X 8 Torq Thrust IIs, which took about 4 weeks to come in. I then got the MT ET Street Radials moved from the previous rims to the new ones, and put them on the car. I was much more pleased with the appearance, and just recently put the car back down on the ground on 4 tires. The tires are now nicely located in the wheelwells, and the size is somewhat concealed.

Here is a similar view to compare against the 4.5" BS picture above.

Another view with 5" BS...

Oops, same picture...here is the one I intended.

I run 275/60/15's on a 70 Cuda with Torque Thrust wheels but I chose either 4.75 or 5" back space. Don't remember which. No contact.
I am thinking you must a good 1" tween tire and leaf-spring? I have about 3/4"

Edit after reading your post above. Yes the close spring contact area is the rear segments, I thought about intalling the rear offset hangers to gain clearance, but didn't have to for 275/60/15s. In fact, I think it may be possible to fit 295/50/15's with the offset hangers.

So now you are sitting on a set of barely used 4.5" BS TT's?

DrDiff reminded me to check the pinion angle after changing the height, and to my surprise it looks like I will be fine with use of a simple shim. I assembled everything with a 4 degree shim in place, because I was concerned that the upper mounting position would rotate the pinion too far up. However, I hadn't considered that in lowering the rear of the car, that was improving the down angle of the transmission output. Currently the car has the trans output at 4 degree down, and the pinion is 2 degree down, and I am considering changing the shim to a 2 degree shim but I think I will try it as is for now.

In conclusion, this approach allows a 5"Bs wheel to be easily used on an E-body without use of any cutting or welding. The only consequence is that the ride height will need to change some.

Two other interesting tidbit is the weight of the rims. The original AR TT-II rim in 15 X 8 with 4.5" BS weighed 16lbs. The custom 15 X 8 5" BS rim weighed close to 18lbs. This was more than I expected, but I certainly want a nice strong weld between the two halves. In addition, I didn't know up front that AR was not going to dress the weld in any way. At first, I was sort of upset by this, but with the wheels on the car, I can't really see the weld from the outside.

The full side view above gives a good idea of the new stance. Here is another side view of slightly better image quality.

Boy, I wish my car had that much clearance! With the 4.5"BS, I only had 3/4" clearance behind the axle. With the 5" BS, it would have only been 1/4", which I was not comfortable with. Now, with the 5" BS, I have 1/2" clearance.

Each car is likely different, and the tire will also affect it because the widest point of the tire can be a slightly different diameters from manufacturer to manufacturer.

Yes, I have two TT2 15 X 8, 4.5"BS that had tires mounted, and about 100 street miles. I am still debating on whether to hold them for another project or sell them.

Also, I don't think you can use the offset rear hangers with no change in the front hanger, without relocating the spring perch. That was something I was avoiding doing.

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Also, I don't think you can use the offset rear hangers with no change in the front hanger, without relocating the spring perch. That was something I was avoiding doing.

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Yea I think your right on the front segments. But I was able to get the needed clearance so thankfully didn't have to go down that road. I would really like to try some 295/50's with the offsets, but at that point the 8"wheel starts to be a little on the 'too narrow' side.