Why so much lift?

From what I read 31" tires fit a stock TJ. I would need to lift 4" to fit 33" tires and 5" to fit 35" tires. So why do I have to lift 4" to fit tires 2" bigger (Actually 1" bigger if you measure from the hub center) and then lift 1" more to fit a tire 2" (1") bigger than that? I understand needing an extra 1" of lift to go from 33" to 35" but why do I need 4" lift to go from 31" to 33"?  
2000 TJ

you dont have to go 4 in. you can get 2in with longer bumpstops and the correct rim width and backspacing, 15x8 with 3.5 to 4.5 i believe. or you can do a highline hood and run a 33 or 35 with no lift. 33's will fit with no lift at all or hood kit but they will rub  
"WHEN IN DOUBT TACH IT OUT!"

You can't really run a 31" tire on a stock TJ. A stock Rubicon runs 31's, but a Rubicon sits about an inch higher and has a raised bump stop on the axle.

Usually, as a tire gets taller, they also get wider. In order to run a wider tire, you need to also factor in wheel back spacing and bump stop spacing differences.

Wider tires require decreased wheel back spacing to keep the tires from making contact with the control arms, swaybar links, and spring pockets.

Wider tires and decreased back spacing increase what is called the "scrub radius". That is the arc that the outside lip of the tire travels in when you turn the front tires from lock to lock. The increase in scrub radius requires more clearance between the tire's outside edge and the lip of the fender flare.

Finally, it is your bump stops that keep your tires from making contact with your fenders when the suspension compresses. Taller, wider tires require the bump stop spacing to be reduced with longer bump stops. If you have less lift, you'll also have less suspension up-travel before making contact with the new bump stops.

So, if you're willing to live with limited up-travel, you can get away with less suspension lift. If you're not a high speed, desert racer kind of guy, you can get away with less up-travel. It's not unreasonable to run 33's with 2-3 inches of lift as long as you have the right bump stop spacing.  

B.M.