18x10.5 56mm offset with 315/30/18

i called a guy from discount tire direct, he told me this would fit with minimal inner fender hammering...18x10.5 56mm offset with 315/30/18..now i thought it had to be 50mm offset??, but this guy said he did it on his 00' camaro. Maybe because the sidewall is smaller, it wont flex as much, allowing it to fit better?

The 50mm offrset is valid with the 17x11" wheels... so it would be valid for 18x11" wheels. That yields about a 7.93" backspace on the 11" wide wheels like the ZR1/GS offsets. With 315/35 on those wheels some people need to hammer the inside fenders. I did.

With a 10.5" wide wheel, and a 56mm offset, I calculate a backspace of 7.91". This is always an "approximate" number, since the actual design of the bead retaining lip on the wheel is a part of the backspace calculation, and this is a variable number.

So, with a 10.5" wide wheel, a 56mm offset fits it in close to the inner fender, exactly like the 11" wheel with a 50mm offset. The outside edge of the 10.5" wide wheel would be 1/2" deeper in the fender than the 11" wide wheel.

If you went to a 50mm offset on the 10.5" wide wheel, it would move it toward the outside of the fender about 1/4", meaning the outer edge of the wheel would still be 1/4" further inside the outer profile of the fender.

I would think 50mm offset on an 18x10.5" wheel would provide a setup that takes NO hammering, and a 56mm offset would simply take SOME hammering, just like those of us with the 11" ZR1/GS wheels.

excellant, thanks for the help

basically the backspacing is the same, i did the calc also...but when u consider using the same size tire..315..and 1/2" smaller rim 10.5 (front side is 1/2 smaller)....isnt the tire going to be pushed 1/2" in towards the inner fender (backside) because it doesnt have that 1/2" on the front anymore?? so really althought the backspace is the same...the tire isnt shrinking because of it, and will be forced 1/2" closer to the inner fender than the 11" wide wheel. The tire doesnt give a rip about backspacing so if you take the center line of a 10.5 wide wheel and a 11" wide wheel the tire will sit closer to the inner fender by 1/2" so really this prob wont work????

But the tire does care about wheel width... put the tire on a narrower wheel, and the section width will no longer be as wide as it was on the wider wheel. The actual bulge in the tire might increase slightly, so that would add to the backspace problem, but not by as much as it seems. You will get a much larger variance in the same size tires from different manufacturers. A BFG Comp T/A 315/35-17 Drag Radial appears to be much wider than a 315/35-17 Nitto 555R Drag Radial.... at least people tell me that's why the Nitto fits on a 9.5" wide wheel without looking "bulgey" and seemingly not causing undue stress on the tire. Note that when tire manufacturers quote the dimensions of their tires... section width and diameter.... they tell you what width wheel they took the measurement on, because those two dimensions will not be exactly the same if you use a different width wheel.