'03 Liberty Renegade.
We also have to consider the differential ratio is only part of the story as the other factor is what gearing is available with the transmisssion. If you’re running a ZF 8 speed, a 3.21 ring gear will have a phenomenal launch due to the low gearing in the trans.Jeeps have come stock with many different gear ratios and different tire sizes over time. If you have had the opportunity to drive your Jeep in the unmolested state with stock gearing a tire size and are happy with the performance it is easy to decide on the correct gear for you. Run an RPM calculator and put it back into the same RPMs at the same speed as closely as possible. Like so close that you do not have to re-calibrate the speedo.
On a 4.0L I agree that 4:10s are 31s. 4:56s are 33s. 4:88s or 5:13s are 35s. That puts it close to a Jeep that came stock with 3:55s or 3:73s on stock tires. On a 4 cyl I would go higher.
I prefer to have a Jeep that is more fun when pressing on the skinny pedal.
Loading an engine harder in lower RPMs causes more cylinder and piston wear than spinning it a little faster IMO. I get your idea but you have to look at the big picture and cylinder pressures and their effects on piston loading. It is highly debatable for sure. I think that there is a middle ground. This is the very reason why some strokers in a round about way have short life expectancies.
I KNOW that if I drove your Jeep I would say it has NO giddy up and go. 33s on 3:07 gears would be a complete pig to me. It just would. I have driven a lot of Jeeps as a Jeep mechanic. Some are way more fun than others. Your gearing and tire size is no fun. Your gearing and tire size are for sure hurting your performance and your fuel economy.
You are only using 4 gears. It hurts your economy. The easiest way for me to say this in a few words is that vehicles went from 3 and 4 speeds to 5 speeds with OD. The onto 6, 8, 9 and even 10 speeds with OD. The point is, the closer the gear ratios of the trans, the more gears that it has, and the more time you spend in the optimal power band, the better.
It is all subjective and there are going to be numerous opinions.
I know that I said all of this. My new to me JKU has 3:21 gears. That number says pig all over it. It performs well. I am happy with it. In fact, I LOVE IT!
As far as driving a brick on the highway at 80ish plus, you are not going to get good economy. It is not aerodynamic at all and the wind drag on a vehicle goes up exponentially over about 50 MPH no matter what you drive. I climbs more exponentially in stuff that has flat glass in the front. (JEEPS)
I remember when we switched from 545RE to the ZF and learned that a ZF 8 speed with a 3.21 differential has a much lower effective ratio than the old 5 speed with a 3.91 diffy.