What's the best way to measure for tire size.
I came up with: Driver side, 6-11/16 from face to leaf spring, 4-3/4 from face to fender. Passenger side, 6-5/8 face to leaf, 5 from face to fender.
I'm putting 18x9 Bullitt wheels on the rear, I think the 265/35/18 will fit. The fenders are untouched stock, i have no problem w with modifying the wheel well.
What year Mustang had the right back space?
Looking at your other posts, this is for a '72 Swinger with a 8 3/4 and LBP axles right? And you're using a Dr. Diff 1/2" spring offset?
If that's the case your measurements make sense. Looks like the axle may be shifted a bit to the driver's side and the bodywork on that side is a little tighter. On my '71 with the stock spring locations and a BBP 8 3/4 I've got 5" on both side from the mounting face to the quarters and 6" to the spring on one side and 6 1/8" to the other, so that's pretty much in line with yours considering the 1/2" offset and some body variation.
On the tightest side (drivers) you've got 11.4" of space to work with. I try to figure on a 1/2" of clearance to the springs and the quarters. You definitely need a 1/2" to the quarters, you can run it a little tighter to the springs but it's nice to have a little wiggle room because section width specs and real life tires mounted on rims aren't usually exactly the same. So, that's only a 10.4" wide tire at the section width. 265/35/18's vary more by section width than any other size I've seen, from 10.3" all the way up to 11.1" depending on the tire. That's crazy, and I only looked at 4 different tires. Basically some manufacturers make a 265 that's pretty much the same as a 255, and others make a 265 the same as a 275. So, you absolutely have to check the section width of the tire you want to run. You could have a tire that fits all the way up to one that isn't even close. Most 255's are 10"-10.4" wide, most 275's are 10.9" to 11.1" wide.
Looking at your measurements, I don't think I'd actually try to put anything wider than a 255 on that car as it sits right now because of the clearance on the driver's side unless you want to trim the quarter lip back. Typically you can gain between a 1/4" to 1/2" by cutting the quarter lip back to the spot welds with the wheel tub. You also might try loosening up your U-bolts and shifting the axle on the springs to see if it'll square up a little better. Otherwise your driver's side is going to limit your tire size significantly. On my car I have 10" to work with and I don't have an offset and haven't cut anything.
Backspace to center an 18x9" on your car should be close to the 6" mark. Assuming a 1/2" clearance and a 1/4" of tire overhang past the rim lip (10.5" section width) the backspace for minimum clearance on the passenger side is 5.875". So I'd be looking for a 18x9 with a 6" backspace and not worrying about ending up at a 3/8" clearance on the inside. I tend to err on the side of slightly more backspace just because you can correct with a thin spacer if you need to. Obviously if you run a wider section width you'd need less backspace to clear the springs, but keep in mind that will push the tire closer to the quarters. On the driver's side you don't have any extra room that way, so I wouldn't go past a 10.5" section anyway.
So, if I were you I'd try to shift the axle on the springs and see if that helps any to balance the sides. Then I'd look at the quarter lips and decide if I really wanted to cut them, and how much. Then I'd see what that gets me for that spring to quarter lip measurement, and look for section widths that are about an inch less. Most 70+ Darts with a 1/2" spring offset can run a 275, but the way yours is measuring out right now I wouldn't go past a 255/40/18 unless you do some trimming.
