I know this is mainly about 4.0s but I check the mpg of my '97 2.5 recently. It was 15.5 with 4.10s ax5 and a 34 ltb. I'm about to swap to 4.88 so I hope that helps it. I was getting 14.0 before swapping a 4.0 throttle body onto it. Probably 85% city 15% hwy.
My guess to answer your question is that later models got a number of upgraded parts and were often more option-laden then earlier models. Extra weight from options, especially ones that affect aerodynamics, could play a role.
2011 Wrangler Rubicon, Zone 3" lift, 3/4" spacers, SRC Front and Rear bumpers, Bushwacker flares.
Yeah I remember with my '97 TJ, on a full tank I'd get to 350 miles on the tripmeter before the low fuel warning. Then I'd fill 17 gallons almost every time. That was with those Grizzly wheels and stock tires, 3.07.
The '98 with 3.73 and 31's would get to 260 and I'd put in 17. That was so consistent for years.
With the '06...let's just say I'm just lucky I don't have to drive too much.
The 6 speed manual isnt much better than the 5 speed as far as the overdrive goes. Just has a taller first gear
I just meant around town, it seems I can fine tune the driving a bit more with the six, for ex. I notice I can take a corner in 3rd gear very easily without lugging. In my 5 speed I was at higher RPMs because I'd be in 2nd. No way I could take a corner in 3rd, without lugging. Just thought keeping RPM's low would help with mileage.
I try to drive with egg between the foot and gas pedal philosophy, it doesn't seem to matter.
My 05 LJ auto w/stock tires/wheels, minimal mods and 24k miles gets 15 mpg +/-1 around town no matter how I change my driving habits (within reason). The only thing that has seemed to make a impact is that I am consistently closer to 16 since I found a local go juice station that is ethanol free. Seems to run smoother at idle as well, but I have a vivid imagination.