hey im buying tires and wondering if a 275/40/18 will fit on the back 2?

 

The tire will be taller by about 8 mm. Width is not an issue. However, I assume you're running a wider rim. A 275 will not properly on an 8" rim. I was running 285/35-19's on a 9.5" rim on a '99 XJR.

 

I ran 275-35x18's, that's the official Jag max size on standard XJR rims

 

Then Jag is wrong. Any tire manufacturer will tell you the minimum recommended rim width for a tire with a 275 mm section width is 9 inches. The delta between an 8" inch rim and 275 tire is too great for the sidewall to properly function. The standard 255/40-18 is really pushing it.

 

I have BFG KDW2's on my XJR in 255/40x18, I've run 275/35x18's on the rear for 2 sets.
I own a Mustang running 275/35x18's same tires on 9" wide rims,
Both sets of tires look OK on both rims, and I use a reliable tire dealer who will not mount tires outside the manufacturers specifications.
What are basing your info on?
I used to run a wheel mfg plant and had specs on max width of tire on a given rim, I cannot remember seeing that these numbers are out of spec.
I think we need to do some research on this.

 

I am basing my opinion on being an automotive engineer for close to 30 years, where over 10 of those years was spent on chassis and suspension design for a F1 team. On passenger cars the section width of a tire should not exceed + or - 125% of rim width for proper function of the sidewalls, which include heat transferrance, proper load distribution, flex characteristics, etc. Believe it or not, most tire dealers don't know a whole lot about tire design, etc. I'm sure you've seen some ridiculous tire and wheel combinations mounted by "reliable" dealers. I'm not going to get into a "he said, she said" joist on a forum, but I'm telling you what I know from years of experience and education.

You obviously can do what you want, but I would never mount a tire wider than 255 mm section width on a 8" inch rim and that's pushing it.

 

I've never heard of the + or -125% rule, based on your past, I think you know your subject, this is exactly what the forums are for. now you've given me a new spreadsheet to create, so I can play with tire sizes and rim width.