Our family has owned 4WD vehicles for three generations. A trend began when my folks traded the '56 VW bug toward a new 1964 Jeep CJ-5. With a curb weight of only 2,500 pounds and powered by a 134ci (2.2L) four-cylinder engine, the CJ sounds much like today's economy cars. When operated between an idle and 2,000 rpm on dirt roads and crawl speed trails, that F-head engine ran all day on a 10-gallon tank of gasoline. At highway speeds, however, we rarely realized 12 miles per gallon -- a common statistic in the 24-cent-per-gallon era.
Since the late '60s, six other Jeep vehicles have adorned our family album--plus two -ton Suburban 4x4s, two Ford F-trucks, a pair of FJ-40 Landcruisers, and our '05 Dodge Ram 3500 4WD diesel. The '50 CJ-3A (134ci L-head) was a restoration project in 1969. An '83 CJ-5, powered by a 151ci four-banger, became an Off-Road Magazine project in 1989-'90, gaining a 4.2L six and an NP435 four-speed transmission. Our ultra-clean '87 Grand Wagoneer with a 360ci V-8 was a rare find in the mid-`90s. The '55 CJ-5 project (featured in my Jeep CJ Rebuilder's Manual: 1946-71) began with a blueprinted 134ci F-head then earned a 231ci Buick V-6 swap. Our '02 3.7L Liberty has stayed within the family, even after the '99 4.0L XJ Cherokee project grabbed center stage.
The 60hp CJ-3A was a 12-mpg-highway vehicle on a good day. The 151ci four- provided 16.5 mpg highway in the '83 CJ-5, while the 4.2L six yielded slightly better. The '87 Grand, tuned properly, actually did well at 14-15 highway mpg, and the '55 CJ with a 231ci V-6 and two overdrive gears now achieves 22-23 mpg highway.
The hefty Liberty struggles between 18-19 mpg, and the XJ Cherokee, with huge bumpers, a winch, 6-inch lift kit, and big tires, gets 18 mpg on-highway--with a lot of coaxing.
None of these 4x4s qualifies as a true economy model, yet in fairness, no economy car could offer the kind of utility and backcountry access that these Jeep vehicles have provided. Today, however, with fuel already exceeding $4 per gallon for our Dodge Ram diesel (a fill-up being $136 or more at that rate), this is hardly the time to leave mileage out of the equation. You might wonder why mileage varies so much between vehicles--engine and chassis designs are clearly the reasons.
Physics and Fuel Appetites
The "under-square" bore/stroke 134ci L-head in the '50 CJ-3A was 1930s engineering. In WWII, this low-compression engine powered generators and a variety of equipment requiring low fuel consumption. In the Jeep 4WD -ton capacity truck, though, with 5.38:1 axle ratios and 30-inch diameter tires, the 134ci engine droned at a peak 3,600 rpm or approximately 60 mph. Notably, such speed pushes the envelope for a stock CJ-3A. The vibration, bias-ply tires, 9-inch drum brakes, and squirrelly handling quickly encourage a decrease in throttle.
The '55 and '64 CJ-5s shared the much-improved 134ci F-head Hurricane four-cylinder with overhead intake valves. The overhead intake valves were grossly oversized--2 inches in diameter, the intake size for fuel-injected small-block Chevy V-8s. Improved performance came at the price of fuel efficiency, and 72 horsepower still required very low axle ratios like 4.27, 4.88 or 5.38 in the CJ models--which further hampers mileage.
The OHV 151ci four-cylinder could be considered the first true "economy" engine in a Jeep 4WD vehicle. AMC borrowed Pontiac's Iron Duke inline, a rock-solid pushrod OHV engine designed for the late `70s GM economy cars. Popular in G.M. J-cars of the `80s, this engine served well in the '79-'83 CJs despite the noticeable power-to-weight deficit of these light utility vehicles. As a result, models like our '83 CJ-5 often gain mileage with a swap to the 4.0L or 4.2L inline six, due in part to better torque at highway speeds. AMC's EFI 2.5L four-cylinder made somewhat better mileage than the sixes.
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.
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.
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.
My strategy is to consciously feather the accelerator pedal and avoid deep throttle, jackrabbit starts that get the turbo whistling. Transmission upshift points are below or at the torque peak rpm (1,600) in each gear--unless a hefty load is in tow. Moving nearly four tons of mass from a dead stop to cruise speed is best handled with a lighter foot on the throttle. You can save more fuel here than anywhere else.
When I learned to drive the four-speed '64 CJ-5, my father cautioned, "Drive like you have a raw egg between your shoe and the gas pedal." His generation survived the Great Depression and fuel rationing of WWII. In our era of foreign fuel dependency and peaked oil supply, you can save fuel by 1) coaxing the vehicle to speed, 2) feeding only enough throttle to keep with the traffic flow, 3) executing upshifts at the lowest engine speed without lugging the engine, and 4) taking advantage of the vehicle's forward momentum to reduce engine load. Begin decreasing the throttle pressure as soon as you feel momentum. Once at cruise speed, use only the amount of throttle needed to maintain steady speed. Allow safe passing distances to decrease the demand for fuel when accelerating, and apply the throttle smoothly. You can do this with any vehicle, whether a gasoline or diesel engine.
Find the rpm where your engine provides sufficient torque without lugging. Use this as your shift point when you don't need hard acceleration for safety or a load's sake. Neither lug nor over-rev the engine. Before making mechanical changes, see how much driving technique can improve your vehicle's fuel diet. As the months unfold and fuel prices continue to steer our lives, your street and trail 4x4 does not have to stay parked. Discover the best ways to maximize fuel efficiency and get the most for your fuel dollars. Next month, we're going off-road and saving fuel in the process.