I found this onthe Leaf Sprung Forum:
I emailed goodyear and Mitchelin a few months back to see what they thought was right for the Wrangeler Radial and XZL ranges rather than what others have herd said. The answer came back from Goodyear as being from "ETRTO", which is "The European Tyre and Rim Technical Organisation" so I guess thats the standard industry recomendation based on best expereances. Mitchelin didn't claim an outside source for their info but blatently used the same data.
Info was...
235/85R16 - 6...7J, 6.5J ideal
7.50R16 - 5.5...6.5J, 6J ideal
205R16 - 5...7J, ideal 5.5J Note that means its fine to run 205R16's on SWB rims! Its also no surprise that 7in is aproved as discos left the factory with this rim/tyre combo, even though it does seem a little wide. Don't know wether the rims outer radius profile (what the J means) is important as everything tends to be J now, but unless you have disco, gayspoke, 130, wolf or the new tubeless 5.5in rims you'll probubly have F profile (or L for 1-ton/FC rims? - another subtle difference between them and 130 rims!). I doubt thats too important but its worth knowing as most tyres are now built with J profile tubeless running in mind. If you measure the sidewall angle between tread width and rim width it does give a simular result to above but I havn't been able to see an acurate trend worth using.
When I was playing with my 235/85R16's before putting them on a rim I did notice they were nominally 6.5in between the beads, and it took litttle effort to move it within the 6...7in range, but a lot of presure to squease it down to 5.5in, or wider than 7in.
I'm currently running them on 5.5in rims as I don't have anything wider but I don't think I will for ever (what would be really nice is to get them on tubeless wolfs for my other truck and the 7.50 XZL version on the new tubeless 5.5J's for the SW, but that'd envolve finding a few bargans (I wouldn't say no to some XPC off a new defender someones alloying)). I'm nowhere near their max load or presure rateing (which is massive on the 235 XZLs) so that half an inch probubly isn't to sereous but then without the test data I can't really say.
What is also interesting is the disclamer data that comes with the above info. It basicly says it is in no way regarded as corect and whatever I do is at my own risk. Mitchelin also said the data was for my use only and shouldn't be passed on (which is presumably why they don't provide what I think is a basic part of the spec on their website in the first place - a bit odd, guess they are just really scared of getting suied).
May help
