I need an 1-1/2” to be flush. A little rub at full lock is okay with me. The tires are a little to far tucked in and the stock tires are 11-1/2” so the 13.5 in width would make it perfect if they fit my rims. But 12.5 would only give me a half inch on the outside.
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i like the way you think but it can be quit difficult to nail down exact numbers, how far tires will stick out, if anything will rub etc. actual tire sizes (diam and width) are likely all over the board depending on manufacturer. i hear bfg are usually small, although i cant confirm it but thats what ive heard many times. had 35x12.5r18 razr , 13.2" advertised section width, and they are the widest 12.5 tire ive ever seen, probly nearly as wide as most 13.5 tires. im sure its probly a crapshoot with diameter as well. some 37s are near 37, others probly just a hair over 36. with 35s you shouldnt have to worry about much rubbing no matter what off set you use. for 37s it seems most experts agree to stay between 18-27mm for minimal rub but cutting could be needed, again because not every brand is the same size. buy what you like, dont over think it, and be prepared to trim some plastic or sheet metal if need be