My new favorite tire. (mounting 285’s on the front)

[robert duncan]

One other thing, as you mentioned rear disc brakes. I'm running LS1 rear disc brakes. How about yours?

 

[1986 Buick GX1]

My rear disc brake rotors act like a .25” spacer.
The 10.5” wide wheel pulls the tire away from the frame a little compared to a 10” wide wheel and even more compared to a 9.5” wide wheel.
The backspacing changes to compensate more for the tire (sidewall bulge) than just the wheel itself.

Sometimes it helps me to image the center line of the tire.

If you mounted the 295/40-18 on a 10.5” wide wheel it seems a 5” backspacing would work with stock drum brakes.

If you mount the same tire on a 9.5” wide wheel, to keep the center of the tire in the same place the outside of the tire will move in .5” and the inside of the tire will move out .5”.

The inside of the wheel moves out .5” as well as the tire. When this happens, the inside of the wheel is .5” closer to the wheel center making the backspacing 4.5”.
Even though the wheel moved out the tire itself stays in the same place.

I don’t know if that helps, I usually have to draw it on a piece of paper to have it make sense. I’ll see if I can do that and add a photo.

 

[1986 Buick GX1]

One other thing, as you mentioned rear disc brakes. I'm running LS1 rear disc brakes. How about yours?

I’m changing from SSBC 11.1” kit to a Speedway Motors 11.63” kit with modified brackets for more frame clearance. Both rotors are .25” thicker than the stock drums.
The LS1 brakes are from ‘93- ‘97 Camaro, right? If so the Speedway Motors kit uses the same rotors.

 

[robert duncan]

The LS1 brakes are only on 98 to 02 Camaros. All models. It is the sidewall bulge that is the culprit, and it seems you have avoided that by choosing a thinner tire than I did. I bought my 20" wheels w/ 5" backspace from a guy who was using 295 30s, and he was just using a washer as a spacer. Maybe he didn't need them at all, but when I decided to go wider with the 305s 12" section width), because of tire bulge I had to use .4" spacers to keep the tire from rubbing on the frame. The rear tires you have chosen only have a 11.6 section width, so to give you more room. The 295 40 18s I would like to use have the same 11.6" section width. I may be wrong, but I wouldn't think using a 9.5" wide rim would make it more apt to hit than if I had a 10.5" rim. The section width is not that great. And also, if you buy a wheel with 5" backspace, it doesn't change according to what tire you use. Perhaps offset, which admittedly, I do not fully comprehend, does.

You probably have this tire size site, but here: https://tiresize.com/height-calculator/

 

[1986 Buick GX1]

The LS1 brakes are only on 98 to 02 Camaros. All models. It is the sidewall bulge that is the culprit, and it seems you have avoided that by choosing a thinner tire than I did. I bought my 20" wheels w/ 5" backspace from a guy who was using 295 30s, and he was just using a washer as a spacer. Maybe he didn't need them at all, but when I decided to go wider with the 305s 12" section width), because of tire bulge I had to use .4" spacers to keep the tire from rubbing on the frame. The rear tires you have chosen only have a 11.6 section width, so to give you more room. The 295 40 18s I would like to use have the same 11.6" section width. I may be wrong, but I wouldn't think using a 9.5" wide rim would make it more apt to hit than if I had a 10.5" rim. The section width is not that great. And also, if you buy a wheel with 5" backspace, it doesn't change according to what tire you use. Perhaps offset, which admittedly, I do not fully comprehend, does.

You probably have this tire size site, but here: https://tiresize.com/height-calculator/

It sounds like you’ve got it. Going from the 295/30-20 to a 305/30-20 adds over 3/8” to each side so the .4” spacer would make up the difference.

That tire calculator is a good one, although I just do the math.

The drawing depicts what I would run on my car for a 9.5” rear wheel and a 295 tire at 26” dia. This includes the .25” extra thickness of the rear rotor. This is why these numbers differ from the example in the previous posts.

I suspect the LS1 rotors are the same.

I hope this helps get my thinking across.
[low quiet creepy voice] I see backspacing. Lol

 

[robert duncan]

Oh yeah! This is great, looking for the same thing, that is figuring out what will fit in our limited space, and actually finding an intelligent answer . I have to study all of this. If it's true, that means I don't have to think about having custom wheels made by strangers who live 3000 miles away and pay through the nose for the privilege. All those $800 to $1200 wheel sets they offer on ebay that have 4.75" bolt pattern, offered in wider than 8" wide wheel have 5.25 backspace. Using my experience with my 5" backspace wheels, I thougt -- no way could I use them. Well, evidently, I can. Thankyou for being so good at math!

 

[robert duncan]

Opening up this again, it seems you can use a 5.25" backspace on the rear, with an 18 X 9.5" wheel like those offered by US Mags or Ridler, but you can go no larger than a 255 40 18 tire. I wanted to be able to use 295 40s and they are much wider and taller than 255s which are the largest you can use with a 5.25" backspace, in order to avoid frame or wheel lip contact. You would think that companies that make so many variations in wheels would be able to offer more than one backspace choice.

 

[1986 Buick GX1]

Opening up this again, it seems you can use a 5.25" backspace on the rear, with an 18 X 9.5" wheel like those offered by US Mags or Ridler, but you can go no larger than a 255 40 18 tire. I wanted to be able to use 295 40s and they are much wider and taller than 255s which are the largest you can use with a 5.25" backspace, in order to avoid frame or wheel lip contact. You would think that companies that make so many variations in wheels would be able to offer more than one backspace choice.

You would think wheel companies would make a wider variety of backspacing options, sadly they don’t.

You could always run a .5” spacer with longer studs to allow more room by the frame in the rear for the 295/40-18’s.

It would be nice if they offered the 9.5” wide wheels in a 4.75” backspacing to the work with rear disc brakes or 4.5” backspacing for the stock drum brakes to make it a breeze.

A 9.5” wide wheel would be more likely to fit the 295/40-18’s without rolling or trimming the fenders.

A 10.5” wide wheel with 5.25” backspacing should work with your rear disc brakes to provide more room by the frame for the 295/40-18’s, but would require the fender lips to be rolled or trimmed.
A 10.5” wide wheel with 5” backspacing would provide the same results with the stock drum brakes.

The wheel width depends on whether you like a bulged sidewall or a slightly stretched sidewall.
I prefer a slightly stretched sidewall.