I've been trying to find the sweet spot for my Jeep too, and i've always thought that above 2000rpm's was too much if you want good fuel efficiency - ? I say this because the "shift assist arrow" always pops up at around 1900rpms, which leads me to believe that anything above that is not "fuel efficient" in the eyes of whoever programmed this at the factory, and the service manual clearly states that this is a fuel efficiency feature. So which one is correct?
The correct rpm for fuel efficiency doesn't really exist. The idea that people have unfortunately created in their head is that more rpm = more fuel. It's understandable that the idea came about since you'd think the more an engine spins, the more fuel it needs, but that isn't correct.
Let's take two Jeeps, one of them doing 2000 rpm at 60 and the other doing 2500 rpm at 60. I would be willing to bet money in nearly every single experiment that the 2500 rpm case would get better gas mileage simply because of the power band of the 4.0. It prefers 2500 to 2000.
Now, let's look away from the power band of the engine and focus on the actual physics of the two Jeeps. There are two advantages to gears: one being that it puts your rpm in the "correct" spot to make all of your transmission gears usable, and the other benefit is that it makes things easier to turn. Take a jeep with 33's on 3.07 and another with 33's on 4.88. Put a wrench and socket on the axle yoke and try to turn the nut, essentially rolling the jeep forwards or backwards. The one with 4.88 will be worlds easier to turn, just as if you were riding a bicycle and peddling in low vs high gears. Now the Jeep doing 2500 rpm has both of these advantages. It's in the power band, but it also has the easier mechanical advantage due to the lower gear setup.
The gas pedal is related directly to your foot via the throttle position sensor, no ifs, ands, or buts about it. Your good or crappy gas mileage comes from how much you push the gas pedal down. If you take the top off and notice less gas mileage, then you had to push your foot further down on average vs when you had the top on. That difference may not be discernible to your foot, but it's there.
So, with the two Jeeps I listed, the driver with the one doing 2500 rpm pushes the gas pedal less because of the easier mechanical advantage that Jeep has due to the gears. The fact that you're at 2500 rpm and right in the 4.0's special spot of efficiency is just an added bonus. 2000 and below is lugging, so you should get less fuel mileage doing 2000.
Back to rpm. People think rpm means more fuel but it doesn't. Since the 2500 rpm Jeep is doing less work, the engine computer gives it less fuel. Sure, the engine may be spinning faster, but the difference is that the fuel injectors are being slowed down as to how much fuel they shoot into the engine. People don't understand this concept, they think the engine shoots the same amount of fuel 24/7 and that if you rev the engine up, that same amount of fuel gets multiplied and wastes tons of fuel. Not the case.
It would be helpful if more people understood the concept. I didn't for a while, but I'm regeared and I can tell you my fuel mileage with 31's went up on 4.10s compared to 3.55, and my accelration is way better. I shift at 4,000 merging on the highway and I get 16-17 mpg going about 70-75 constantly.