I think I have done my PHD in research on M/T tyres of late. I've also got a 4.8 GRX and am looking for a set of M/T tyres for trips etc.
These seem to be the issues:
1 - Diamater variance from OEM
2 - Rim size: current 8JJ
There seems to be some thought that you should
not increase the diameter by more than 3% from the OEM without putting strain on couplings, gearbox and brakes. The LA Sport patrol which has been featured in the last few leisure wheels had 305/70/17 tyres in those pics in the dunes. Thats a diameter thats 8.11% MORE than the OEM of 275/65/17. They have changed tyres to 285/70/17's which are 4.99% larger in diameter. This size to me is the best size as it will also give a lift of 41mm from OEM. Thats as much as putting a suspension on. The rim size range on this tyre is 7.5" - 9" which means that a 285 section tyre will be in the middle of the standard 8JJ rim in a 4.8 patrol. The 305 section (and a 295 section) tyre requires a minmum rim of 8". Too narrow a rim (like an 8" rim on a 305 section tyre) will allow lots a sidewall flex when fast cornering on roads. Too narrow a rim will result in a harsh ride and deflation wont work as well.
I spoke to the guys that installed the rims for the 305 section tyres (they put 9" wide rims on for those wide tyres) and he said that they had to shim the rims as they interfere with a weld on the Patrols' hub. bot a bog though and each of those rims cost about R1200.
There are two great websites to do the calculations that I have summarised in the attachment below.
http://www.carbibles.com/tyre_bible_pg4.html
http://www.1010tires.com
After all this my conlusion as to tyre size is:
285/70/17 is the best compromise on size as:
- The increased diamater seems manageable from other posts I've read (even though its more than the recomended 3%)
- It's required rim size of 7.5" - 9" means that the standard 8" is neitehr too large nor to small.
- there seem to be quite a few makes that make this tyre. BFG, MAXXIS, Cooper, Firestone & Goodyear. There are others but these makes are on my shortlist.
QUESTION: Has anyone tried the Firestone or Cooper M/T. In some reviews that I found on the net the firestones performed really well.