A new air filter and fuel...
A new air filter and fuel filter will help performance. With EFI engines, an aging filter will cause poor performance. On a carbureted engine, a dirty filter means a rich, fuel-hungry air-fuel ratio--like leaving the choke partially on. A paper filter cleans very well, but it will clog quickly if you drive on dusty, alkaline trails. For fuel efficiency, check the air filter often.
Peak torque speed is important because that is where an engine operates most efficiently. An engine under load will perform well at its peak torque speed, and our bulked up XJ is clearly under a load at highway speeds. With 33-inch tires and 4.10 gears, the 60 mph engine speed is 2,506 rpm in 3rd gear. In the 0.753:1 ratio of 4th gear (overdrive of the AW-4 automatic), the rpm drops to 1,887. Peak power for this engine is 190 hp at 4,600 rpm and 225 lb-ft torque at 3,000 rpm. Dropping the rpm to below 1,900 at highway speed in overdrive is so far below the torque peak that the engine is both sluggish and lacking in fuel efficiency.
Hefty accessories and a taller, wind-buffeting profile from the lifted chassis exaggerate the problem. Using third gear at 60 mph, the 2,506 rpm is much further up the torque peak curve and, especially on a grade or when pulling, fuel efficiency actually improves. As for running up to 3,000 rpm to reach the 225 lb-ft of peak torque, the engine would feel strong at the expense of fuel efficiency--since the vehicle would be traveling 72 mph at that engine speed in third gear. (Driving 72 mph, in any case, does not support good fuel economy.) Although efficient, the engine simply spins too fast and works too hard to achieve optimal fuel mileage.
4.56:1 gears are worth considering for the 4.0L engine in this 3,900-pound vehicle. The 1:1 third gear ratio would yield 2,787 rpm at 60 mph, and overdrive would drop the rpm to 2,099 rpm. At 2,100 rpm, the engine is somewhat up the torque curve--certainly better than 1,887 rpm. Changing to 4.88:1 gears, the rpm in third would be 2,982, with fourth (overdrive) yielding 2,245 rpm at 60 mph. This would provide the flexibility of peak torque at 60 mph in third gear for toting a smaller trailer up a 6-percent grade. In overdrive, the 2,245 rpm would provide better torque for pulling an accessorized XJ Cherokee on the flat.
Likely, the 4.0L six would do better with 4.88:1 gearing if you plan to use fourth gear overdrive on the highway and 33-inch diameter tires. Light throttle would provide quicker 1-2-3 shifts, and in overdrive, the engine would develop reasonable power. Under heavy load, the 4.0L becomes quite sluggish below the 2,300-2,400 rpm range. For 33-inch tires and maximum fuel economy, 4.56:1 would be my choice. For light towing and improved performance, 4.88:1 and proper driver input could work.
Peak Engine Efficiency and Torque = Mileage
Fuel efficiency is closely related to the volumetric efficiency of an engine and its torque character. Volumetric efficiency is a product of engine design, specifically the bore/stroke configuration, compression ratio, efficiency of the air-fuel flow into each cylinder, and the quality of the exhaust scavenging. A valuable reflection of volumetric efficiency is manifold vacuum: the higher the vacuum at a given rpm, the more efficiently the engine is operating. For building non-supercharged race engines and maximizing power, the mantra has always been, "A naturally aspirated engine is nothing more than a vacuum pump."
Some of us are old enough to remember the "Econ-O-Meter." Sold at auto supply stores in the 1950s and `60s, the meter was simply a manifold vacuum gauge. The gauge had a red and green zone and a sweep needle. Higher vacuum readings were in the green zone. Hard acceleration dropped manifold vacuum and dipped the needle into the red zone. A modern equivalent is the "Upshift" light for manual transmission models or the trendy fuel efficiency LED readouts.