Rotational mass + brick shape + 4.0 = you need some RPM's to cruise efficiently and to provide the motor with adequate leverage. At 70 mph, the resistance against the shape of the jeep is working hard against you. IMO, the 4.0 needs to be at an absolute minimum of 2400 and preferably in the 2600-2800 range at 70 mph. Lower is not better in this case as you've seen with the 42rle lugger. The 4.0 reaches the point of peak volumetric efficiency at about 3100 RPM, so running closer to that is more efficient than lugging it. Regardless, you can't expect to get acceptable fuel mileage at 70 mph due to wind resistance, even if optimally geared. After about 60 mph, the dropoff is sharp.
As for factory rpm limit, not sure what you mean by that. If you're saying you never go past 3k, you may want to do that occasionally. Hell, I probably redline mine at least once a day....that's 5k.
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