Fluids for this TJ Rubicon...
Fluids for this TJ Rubicon are Mopar/Jeep products. Many of the factory engine and gear lubricants now have a synthetic base, engineered for better fuel mileage. Every bit helps, and using the right fluids can make a difference in mileage and increased vehicle life.
An emissions era, carbureted 360ci V-8, the '87 Grand Wagoneer was a nice package. Luxury, mass, trailering capability, power, and decent handling make the model popular to this day--though maybe not tomorrow with fuel costs soaring. For a luxury liner, the J-truck based Grand, with its tall 3.32:1 axle ratio and A727 Torqueflite automatic, was fuel efficient in its day, a testimony to the design and tune of the rugged AMC/Jeep 304, 360 and 401 V-8s.
Why does the boxy '55 CJ beat out all of the others for fuel efficiency--even the Liberty and '99 XJ Cherokee? The '55 was custom-built to excel at highway driving. The axle ratios are a typical 5.38:1 with 31-inch diameter tires. However, there are two overdrives: a classic Warn 25-percent unit at the Model 18 transfer case's PTO point plus the fifth gear overdrive of an NV3550 transmission mated to the 231ci Buick V-6. A light 2,700 pounds curb weight and tall gearing--equivalent to 3.34:1 axle ratios with both overdrives engaged, the '55 CJ-5 has been transformed into an economy vehicle. Of clear importance, the engine spins near 2,000 rpm at 55-60 mph--the peak torque point for this Buick 231ci (3.8L) V-6. Were the body anywhere near aerodynamic, mileage would be even better.
A Pattern Emerges
When looking for fuel efficiency factors, two items come up immediately: 1) the power-to-weight ratio of the vehicle, and 2) equivalent gearing at the axles. Power-to-weight has to do with the engine output and chassis/body design. Importantly, light trucks must be able to perform work. Low (numerically high) axle ratios like 5.38:1 were once fully acceptable for 4WD -ton vehicles designed to crawl off-pavement and run power take-offs on remote work sites.
By 1983, the CJ's 3.73:1 axle ratio for a four-cylinder model reflected CAFE/EPA standards of the era. AMC/Jeep complied with stringent tailpipe emission requirements and the federal demand for better fuel mileage. This, of course, comes at the expense of performance, and rock-crawling or big tires exaggerate the problem. Even worse axle ratios like 3.08:1 emerged in the late CJs and became prevalent with the YJ Wrangler models. Tall factory gearing and trendy oversize tires have inspired the brisk aftermarket sale of ring-and-pinion gear sets.
The Best Gear Ratios for Fuel Efficiency
Generally, lower (numerically higher) gearing means more pulling power, better rock crawling ability, and higher engine speed on-highway. Taller (numerically lower) gearing provides lower engine rpm on the highway with possible fuel savings. For better fuel efficiency, taller gearing would seem right--but that's not always true.
In some cases, and the '99 XJ Cherokee fits this category, the gearing may be too tall. With 33-inch diameter tires, I precisely calculated and restored the factory highway rpm by switching from stock 3.55:1 gears to 4.10:1. This is so close to the OEM engine speed and gearing dynamic that the speedometer needed no correction. "Isn't this smart?" I thought.
The result, however, was that the heavier curb weight of the vehicle--with hefty aftermarket bumpers, a 9,000-pound-capacity Warn winch, and oversize Toyo tires--actually made the engine lug at highway speeds. Some of this lugging reflects the OEM engineering strategy, but with the added vehicle weight, fuel efficiency would benefit from lower gearing--ideally, either 4.27:1 or 4.56:1. Why is a slower engine speed not fuel-efficient in this case? Because the torque peak for the 4.0L engine is at a higher rpm than the highway cruise speed for this vehicle.