1955 CJ-5, the first year...
1955 CJ-5, the first year for this civilian body, was only available with a 134ci F-head four-cylinder engine. The swap to a Buick 231ci V-6 with two overdrives bumped mileage from 14.5 mpg with just the Warn 25 percent overdrive to 22-23 mpg highway with the Warn overdrive and an NV3550 fifth gear. The combination of two overdrives provides an equivalent 3.34:1 axle ratio, just right for 31-inch tires and a vehicle weight of 2,900 pounds.
Other factors that control efficiency include the camshaft's profile, induction system design, and the exhaust system. For racing engines, the "tune" of an engine involves selecting the right camshaft for specific driving conditions: 1) higher rpm power for drags, hill climbs and the Baja, 2) mid-range torque for complex terrain, 3) bottom-end torque for rock crawling, and 4) specialized profiles that meet other driving environments. For a street/trail engine, the overall concerns are drivability, fuel efficiency, and good low-to-medium-speed performance--not a top-end, high rpm horsepower camshaft.
In engine evolution, objectives have changed. The early L- and F-head Jeep engines were long stroke and under-square bore layouts, built for bottom-end torque. A 134ci F-head reached peak torque at 2,000 rpm. The later AMC 232/258ci OHV inline six designs were noted for low-end torque as well. At one point in the emissions era, the 258/4.2L, with its longer stroke than the 4.0L, reached peak torque by 1,600 rpm. This is the same torque rise as a Dodge Ram Cummins 5.9L diesel engine.
When a gasoline engine develops this kind of torque, the camshaft profile usually has good lift and milder duration. There is very little valve "overlap," the cylinders fill and seal well at low speeds. Change this camshaft profile somewhat or shift to a bore-stroke change like "over-square," and the "evolved" 4.0L Cherokee engine does not develop strong torque until 2,400-3,200 rpm.
The modern Jeep engines are simply not bottom-end "stump pullers." Instead, they rely on gearing to meet off-pavement and towing requirements. Lug a 4.0L inline six, 2.5L four, or the new 3.8L V-6 found in the JK Wrangler, and you will quickly discover this shortfall. These engines need higher rpm to pull well. Raising the rpm can be accomplished by lowering the axle gear ratios--changing from 3.55 to 4.10, 3.73 to 4.10 or 4.56 and so forth. Higher engine speeds, however, also raise the amount of fuel consumed. We need to find the point of adequate engine speed--as low as reasonable yet providing sufficient torque.
What Can You Do Today?
If fuel efficiency is a concern, there are driving techniques that can dramatically improve your vehicle's economy. With engines that develop sufficient torque at lower speeds, the trick is to keep rpm in check. For example, the 4.2L or the 4.0L sixes, or any of the Jeep V-8s--and even the JK's 3.8L V-6 or the Liberty's 3.7L V-6--can be operated below their peak torque rpm and still produce good power.
Note: For fuel efficiency, we're interested in torque peak rpm, not the horsepower peak rpm. Horsepower peak speed is always much higher than the torque peak speed. For fuel efficiency, an engine should run at a speed high enough to produce good torque--but only slightly beyond its torque peak speed unless there is a need for high horsepower.
A great example of efficiency is the Cummins 5.9L diesel in our '05 Dodge Ram. Diesel power is always about torque. Torque develops at lower speeds in a diesel engine than a similar gasoline design. The Dodge Ram produces peak torque at 1,600 rpm. Horsepower builds from this point to around 3,000 rpm (redline 3,400 rpm). The best fuel efficiency with this package, a consistent 24.3-plus mpg highway (without a trailer or hefty cargo) is from driving in the 1,600-1,900 rpm range. Above this speed, fuel consumption increases with rpm. Below 1,400 rpm, the engine works harder and will likely use more fuel. (Lugging is very bad for a diesel.)
I talk with many diesel owners. Few enjoy this kind of economy, and driving technique is clearly the reason. One friend bought a Cummins Dodge 4WD with a manual six-speed. Ours is an automatic. His fuel consumption was 17 mpg highway without a load. Asked what speed he shifted the truck, he noted, "3,000 to 3,400 rpm." This would be great for a twin-cam V-6 gasoline engine.