Here’s a great little TJ gear chart to find out what gear ratio would be good for you if you are changing tire sizes. Keep in mind that this is in no way a definitive answer. A lot will depend on what type of terrain you wheeling, how much on-road/highway driving you do, what MPG you are aiming for, etc etc. Also keep in mind that this is a general chart. It does not take into account what transfer case you have.. If your TJ is not a Rubicon you will have a NV231 t-case and 2.71:1 crawl ratio. If you have a Rubicon you have a NV241 t-case and a 4:1 crawl ratio. This has a very big effect when you are wheeling in 4-low! You can get away with a lower gear ratio and still crawl slower than your friends with 2.71:1. On the other side of the same equation you can gear the same as your friends without Rubicon’s and crawl even slower! Another factor to keep in mind is that the tire size listed on the tire itself is not always the actual size of the tire. Some tires in the upper 30′s and 40′s are up to a couple inches smaller than what is listed.
For what I use my jeep for currently, I think right about the higher blue levels would suit me good. For example if I go to a 36″ tire, I would want to be right at 4.88. That would give me lots of gearing for mud and hill climbs, decent power at a higher RPM on the highway, and the usual horrible MPG like I expect out of my jeep (only worse than stock)!
Stock gearing in my jeep is 3.07, and the stock GSA tires are about 28″ in size. 27.7″ x 8.5″ to be exact. That puts it right in the bottom of the yellow area, which gives pretty poor performance on the highway. Add 31″ tires to the mix (my jeep), and the issue is even worse.