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Maximize Fuel Efficiency - Fuel Efficiency 101 - Tech

Gas Cans
Moses Ludel | Writer
Posted June 1, 2009

Part I: Drive More And Use Less Fuel!

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.

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