Changing tire height affects the effective gearing of the axles. In other words, if you have 3.73 gears stock and 27" tires and go up to 31" tires it would be like pushing the 27s with 3.24 gears.

_______?_______..................... =.................. ____Stock Ratio_____
Stock Tire Diameter...........................................New Tire Diameter

The easy way to work that is to cross-multiply the stock ratio and stock diameter, then divide by the new diameter. That will give the effective ratio of the stock gears on the new tires.

So, 3.73 x 27 / 31 = 3.24

This effective ratio will lower the engine RPM for a given MPH, which will put the engine low in its power band. You can either get around this by downshifting the transmission, or changing the differential gear ratio. Many people that don't change the diff ratio effectively lose the OD gear of the tranny, because they have to be a gear down to maintain any engine power.

Basically, it is best to change the gear ratio. You can either choose a set of gears that will give the equivalent effective ratio as stock, or go a little to either side depending on what your needs are. Lower gears (numerically higher - ex. 4.10s vs 3.73s) will give better acceleration and better low-speed crawling. A popular trick to making the old factory muscle cars fell more muscular was to change the gear ratio for the better 0-60 time.

It is easy to modify that first equation to get a way of determining the suggested new ratio for a new tire size:

....Stock Ratio .............................. =.................. _______?_______
Stock Tire Diameter...........................................New Tire Diameter


So, (3.73 x 31)/27 = 4.28.

I've been using a stock diameter of 27" (215/75R15), but with no modifications a BII will fit 29s (235/75R15) and turn them fine with 3.73s. Using 29 instead of 27, you get 4.11 for the suggested gear ratio. The cheapest way to pull that off is to get some junkyard factory axles with the optional 4.10 gears.

Suggested gear ratios:
stock - 30" : 3.73
31 - 32: 4.11
33 - 36 : 4.56
37+ : 5.13

Now that is in a perfect world where only effective gearing matters. The fact is, there is extra air resistance, weight, etc. making those bigger tires harder to move. Therefore, at 35" and up, 5.13s are a good idea. It will give 2.9 V6s the upper hand against those big tires. One caveat, the Dana 28 front probably won't survive 35" tires very well. A D35 from a 4.0L RBV (or any other Dana axle swap) will give a more readily available 5.13 gearing option along with all the strength benefits.

Engine swaps have the approximate effect of a gear change. Figure a 4.0 V6 is one notch and a 5.0 V8 is two notches up in grunt from a 2.9. Therefore, a healthy stock 5.0 will push 33s no problem, but the RPM will be low. This results in funky shift points (with an auto, it will bog down on hills and then when it automatically downshifts you will take off like a fricken rocket) and decreased control of low speed maneuvers (like trail running). You don't want to be nursing your way around a corner and have it suddenly jump forward into a tree because you haven't got enough throttle control.

Class dismissed.