Largest tire on stock rim

[KJE]

Got the new edition to our family at the beginning of this month and was initially planning to run 18” aftermarkets and ordered a set and dry fitted a tire, but they just didn’t look good in the flesh and since then, haven’t been able to find a rim I really liked better than the stockers, so now looking at keeping them but with wider tires.
I don’t want to use spacers as had some issues in the past and was wondering if anyone knows if the following tire sizes will fit the stock 21” rim – 305/40/21 front
and 315/40/21 rear.

 

[zdubyadubya]

the widest you will want to go on a 9" rim will be 285. Even then, the tire mfgs recommend a 9.5" wheel for 285. of course you can find folks that go wider but its generally a bad idea.

whats also a bad idea is running two different diameters of tire. a 315/40(r) and 305/40(f) setup will not work.

 

[KJE]

Agreed. I usually work on trucks where I have more rubber on the sidewall and also forgot to take into account the shorter the aspect ratio, the closer the tread width has to match that of the wheel. I’ll just keep looking for rims

 

[zdubyadubya]

look at @blaster9 's rig. i believe he has fitted the largest off-road tire that you can stuff in these things and its a 265. you could maybe go to 285 with a lower profile tread but its still gonna be dicey.

i really like this tool for mocking up fitment. you can't increase OD AND offset too much without rubbing so i would stick with a 30.75 OD tire and push your outer clearance out up to a max of 31.75mm and inner clearance is limited to like an extra 12mm over stock. obviously, if you go with a 31.5" tire like he did, you will need to bring that offset in a bit.
Online Wheel and Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

 

[KJE]

Appreciate the suggestions and the link. Unusual to find a UK site offering this information. I used this site
https://www.wheel-size.com/calc/and also complimented blaster 9 on his work, planning to go a similar route at the start. At one point had even considered getting the 18" PIU unit steelies and widening or alloy equivalent but in the words of U2 "I still haven't found what I'm looking for"