This is probably really obvious but
Why is it that a stock 3rd gen p/u can fit 31's no problem, but lifting it 4 inches only gives you the ability to run 35s with some rub? If youre getting an additional 4 inches between the axle hub and the fender, shouldn't that be enough clearance for 31+4+4=39s? I know that it just plain doesnt work that way, but does anybody have some geometric knowledge on why that is?

 

Scrub radius and the fact you have a firewall.

 

Also because the 35's are usually 12.5" wide. For comparison, you can fit a 35x10.50 with almost no lift