Regearing:

What is regearing? Regearing usually refers to changing the ring and pinion gears in your differentials to a different gear ratio.

Why? What do gear ratios do? Gears transfer torque, and a gear ratio describes how a set of gears transfers torque through it. Imagine two interlinking gears, if one gear had twice as many teeth as the other, this is a gear ratio of 1:2 (input:output), one revolution of the larger gear (input) results in two revolutions of the smaller gear (output). In other words, the output gear rotates at twice the rate of the input, but with half the torque.

How does that matter? When your Jeep is geared at the factory, they choose the gear ratio based on many things, including what size tires are on the Jeep. If you then change tire sizes, the amount of effort the engine has to provide to make them rotate changes. By changing the gear ratio, you shift the amount of "advantage" the engine has over the wheels. This can make it a little easier to turn large wheels, at the expense of maximum speed. This can make the Jeep feel like it's got a bit of its power back and make it more enjoyable to drive.

Do I have to regear for any tire change? Regears can have many benefits, but only really need to be considered if you make a large leap in tire size, or if you change tires and notice that the Jeep feels a bit more "sluggish" than you like now.

This is complicated, how do I know what gear ratio I want? Research is important here, but you can find gear ratio estimates by using charts like This One or calculators like This One. Neither of these options can take everything into consideration though, so it can be a good idea to try to find forums or online support from people who have the same Jeep as you, and see what has worked for others. Here's a forum for regearing TJs for example. If you call or go to a differential shop, they should be able to help you decide too. If you want to know what kind of Gear Ratio came in your Jeep, you can look up a buildsheet, or look on your differential covers, there is usually a tag or marking that will say the stock ratio on it. If all else fails, you can also open the differential and manually count the teeth on your ring and pinion and calculate the ratio yourself.

Is this something I can do myself? While it is technically possible to regear at home, and some people have done it, it's a very precise job that needs somewhat uncommon tools for the average Jeeper. This is one of the few upgrades we recommend taking to an expert unless you're very experienced with this kind of thing. If you're set on doing it yourself, take your time and make sure you do plenty of research, messing up a differential can be more expensive than the regear would have been.

Further reading: Why change gear ratios? (External Link)

Bonus Clarification: Techinically the ring and pinion are a part of the Final Drive Assembly, not the Differential Assembly which is the Spider Gears. The term Differential is frequently used to refer to both, but it can help ease confusion to use the proper terms when necessary, such as when talking to your mechanic, so it's worth knowing. For example, changing your differential gears may refer to replacing your Ring and Pinion, or your Spider Gears, so saying you want to change your Final Drive gears specifies the Ring and Pinion.

If you're looking into Regearing your axles, it's a good time to consider installing Differential Lockers. The work needed for a Regear closely aligns with the work of installing a locker, so you can save some money by doing both at the same time.