well i have 35" tires with 6" of lift (3.73 gearing, 4.0 5 speed) and my rig stays at about 1700rpm at about 50mph and about 1800 at 60mph.. I have a custom 3" intake, bored out throttle body, 2.5" catback exhaust (full with 2.5" tail pipe), electric fan conversion (i think thats about it on the performance side).. My rig holds speed at such low rpms with no problem, It runs very smooth so either people are wrong, or they are full of it, or repeating what other have said, or the mods actually make a difference and the people saying otherwise are still wrong.
Also cant forget i was barely getting 180miles per tank stock before the gas light turned on, now i go way past that. I just filled up today 271 on the odometer when the light first turned on and i filled up few tenths above 16 gallons.
In addition to keeping it tuned up I loose the back seat when possible, drive 60 mph (your crazy going faster then that in a Wrangler anyway), and keep the tires inflated to around 41 psi. I get about 23 highway.
Here in Germany you cant do anything under 75 on the Autobahn with out going under the min. speed limit. Semi-trucks will pass you. I average 85- 95MPH and 9MPG. With a 2" lift 31*10.50.R15 M/T inflated to 45PSI, Dynomax Superflow cat-back, aFe cold air intake, Edge Trail Jamer chip, and a bored out TB.