Confusion with backspacing and tyre width!
 

Hey guys, looking to put 255/85r16s on the jk with new rims.

Really want to keep the tyres within the fenders guards.

I figure a 5" backspaced (+12 offset), 8 inch wide rim with the 255's will keep me within the guards. I worked this out based off knowing a 285/75r17 tyre on a stock jk rim with 6.25" of backspace will sit on level with the guard. So a 285 wide tyre is 40mm wider than the stock 245 tyre. So we can add 40mm of width and still stay in the guard. Losing 1.25 inch of backspace on the 16 inch rim and 10mm wider tire will put us at 35mm wider and within the guard, is this correct?

Would appreciate if anyone could clarify or give advice

https://tiresize.com/comparison/

Check this site and it will tell you everything about what you want to change

Remember tyre width increase on both outward and inward of the centre. 40mm wider tyre on the same rim only sticks out 20mm further.

I had 315's on, 8.5" +18 wheels (same offset as 8" +12) and it was flush with the factory flares, 255's will 100% still be in the factory guards with that offset.

your stock offset rim is +44.3mm, the stock fender clearance of my stock 255/75R17 ruby tyres on the stock rims was 20mm (measured). That means you can go to a +24.3mm or higher offset in a 255 and be flush with the fender (in theory).

Practically it depends on the actual tyre width which vary from the stated width between manufacturers. I think it would be very close.

Another consideration is track width increase laws (50mm). In theory police could fine you for exceeding them but I don't think they could ever be bothered measuring your track. Also insurance if you give those blokes an out I'd be surprised if they didnt take it.And if you think you might need to get your car roadworthy'd to sell it in the future hang on to your stockies or go for an offset of +20 or higher.

I now run 285/75r16 kuhmo mt51's on +40 offset 16x7 steelies and the new fender clearance is 2mm

Thanks for the feedback there guys. The tire size website is pretty awesome. Going to look for wheels with around +30 offset. Figure it's worth keeping it all legal etc! Thanks for the help

Will 16" fit your JK? I thought all but the very early JK's needed 17" to clear the brake callipers.