Tires more than an inch or two larger than stock may require you to change the ring & pinion gears in the differentials of both the front and rear axle in order to keep your engine running in the optimum power range - especially when off-road, going uphill or driving on the freeway.

Increased tire diameter increases the torque loads on the axles, u-joints & drive shafts. For significantly larger tires it is usually necessary to upgrade these components, especially where traction aiding devices such as lockers or limited slip is used, or you may risk breaking one of these components on the trail.

Bigger tires are heavier tires. This increased rotating mass and the increased leverage provided by the larger tire often call for increases in braking power: changing brake pad materials, rear disc brake conversions, beefing up the master cylinder or even switching to larger disk and calipers are all options. Driving a tall, heavy, modified short wheelbase Jeep with 35” tires and stock brakes in poor condition at highway speeds can be a recipe for disaster!

Increases weight caused by the larger tire and any axle or brake upgrades also increases unsprung weight. This does not matter much on the trail, but can make the ride a higher speeds much more harsh. When mounting the tire you will find that it is often much more difficult to balance the tire correctly. This is due to the extra weight of the tire, the increased rotating inertia of the tire and manufacturers tolerances on larger tires. It can take quite a bit of weight to balance a large tire and many shops that do not routinely handle tires of this size may not do a good job.

EFFECTS OF WIDER TIRES:

Tire diameter is not the only issue - tire width is important to. Wider tires increase footprint which can be important for traction or increased flotation. The tradeoff include that wider tires can also require extended fender coverage in many states and may require changes in wheel width & offset to work properly. On stock offset wheels the tires may be mostly covered by the stock fender flares but rub on suspension components when turning thus reducing effective turning radius. The incredibly tight turning radius of a jeep is one of it's important advantages on the trail. Wider wheels or wheels with increased offset move the tires outboard and can greatly reduce the rubbing when turning but make the vehicle wider. This increased width may be an advantage for stability if you drive a lifted vehicle or a disadvantage of you are squeezing down a trail better suited for a Samurai or a quad. Lastly narrower tires can sometimes be an advantage where the best traction is located just a few inches below the surface (such a shallow snow or mud) and the decreased footprint and increased pressure aids the tires in getting down to where it can get a bite. (http://www.off-road.com/jeep/2001/TechWhatFits.html)

I asked Moses specifically about adding a lift kit to the TJ. He said that there's a trickle down effect, because some people feel there's a need for them, but there are a lot of pitfalls. Adding a lift kit stresses the power train and compromises safety and handling. First, lifting the vehicle increases the center of gravity, which is a liability (remember 60 Minutes?) To get around this, you need to increase the track width of the vehicle, which often comes with the bigger aid wider tires. Some people do this by using wheels with more negative offset. However, this increases the load on the wheel bearings. Also, he said a lot of late model vehicles use electronic speed counters to monitor the antilock brakes and shift points, so upping the tire size changes this (since it's changing the effective final drive gearing). Using really big tires screws up the axle gearing for highway usage, to the point where you need to change axle gear ratios. It also affects the driveshaft angle and could cause problems with the alignment, since the control arms are in a different position. And, the manufacturer of the vehicle can't honor the warranty on problems caused by a bad lift kit.

The moral of the story is that lifting a vehicle has to be done properly and safely. The only real use for lift kits is in off-pavement usage. If you always tow your Jeep to the trails on a trailer, you might not have to worry about a lot of these problems, but you might want to think twice about lifting a daily driver. He did recommend the Warn Black Diamond lift for the TJ, since it addresses a lot of the safety issues and fixes the steering geometry and caster angle. (http://www.visi.com/~tam/campjeep/accessories.html#liftkits)