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Building A Fuel Efficient Jeep - Project Mileage Master, Part 1Building A Jeep To Drive From the August, 2008 issue of Jp By Pete Trasborg
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It is an accepted fact that the more built a Jeep is, the worse the gas mileage gets. Often we build our Jeeps up to do so well off-road that we can't justify the cost at the pump of driving them on-road anymore and end up buying a second vehicle to drive day to day while the Jeep sits languishing by the side of the house. We recently bought this '98 Cherokee for $1,000 to drive back and forth to work. It has the 4.0L, AW4 automatic transmission, and NP242 transfer case, and we quickly fell in love with being able to get 340 miles out of 18-19 gallons of fuel. It turns out that for a Jeep, a stock Cherokee gets great gas mileage. The problem was that because of the mileage the Cherokee got, we kept taking it places and wheeling it, which, since it had no skidplates, a low-slung stance, and other stock-Jeep limitations, meant there was always a trail or obstacle impassable to our stock XJ without damage. We got to thinking: What if we could build this Cherokee to do better off-road, while at the same time retain its on-road manners and mileage? Thus Project Mileage Master was born. One of the reasons we were... One of the reasons we were able to pick this Jeep up for a song was because it had been in an accident. It was hit in the rear quarter, spun, and went up a curb. In the process two of the stock rims were broken, and another one bent. So in order to drive it, we mounted a set of well-used 29x11.50R15LT Super Swamper SSR tires on Dick Cepek DC-1 rims so we could drive it home. We are going to take a two-pronged attack to achieve our goals. We are going to add all the stuff that an off-highway Jeep needs, such as lockers, armor, a lift, more aggressive tires, recovery equipment, and the like. All this stuff adds weight, can affect efficiency, and can lead to increased fuel consumption through added drag, friction, and rolling resistance. So, we are going to keep an eye on what stuff weighs as we add it and see how it affects fuel economy. The other side of the equation is that we think if we can increase the power output of the engine and increase efficiency of the drivetrain overall, then by putting more power to the ground for the same amount of fuel consumed we can possibly actually increase the mileage of the Jeep as we go. So, we are going to be testing power adders and for each one note what change in mileage there was, if any. After each modification we make in an effort to get more power, we will dyno it for real-world test results on each modification. After each change, we are going to put at least a thousand miles on it-but in most cases 2,000 or more miles-and calculate the average fuel economy as we go. Even if you don't have an XJ, a lot of the things we discover along the way will help you in building your Jeep to become a better wheeling machine while not breaking the bank every time you pull up to a gas pump.  Since we had no idea of the...  Since we had no idea of the maintenance records concerning this thing, we gave it a tune up. Plugs, wires, cap, and rotor all got replaced. The button in the cap was worn through the brass and the gap in the plugs varied 0.060- to 0.090-inch. We went from a solid 16 mpg on the highway to 17-18 mpg. The next weekend we replaced the engine oil, transmission fluid, transfer case fluid, and the fluid in both differentials, but noticed no difference in economy even though all the old fluids were filthy.  At this point, we took the...  At this point, we took the XJ on its first wheeling trip. One of our buddies following us noticed that the rear wheel was vibrating really badly on the highway. We didn't notice because of how beat the tires were. But when we got home, we found that the axleshaft was bent and almost 3/8-inch out of true. We swapped in a junkyard shaft, but the damage was done: Our half-bald Swampers had had it. So, we got a set new set of 30x9.50R15 Goodyear MT/Rs and mounted them on our DC-1 wheels.  Like nearly all older Jeep...  Like nearly all older Jeep 4.0L tubular exhaust manifolds, this one was cracked. We were thinking that since only half of one tube was still welded to the collector, maybe our O2 sensor was getting bad readings. We welded up as much as we could, but there was about a third of the total area we couldn't get to between the oil pan and the manifold. We saw no change in mileage from the repair.  On that first drive, we got...  On that first drive, we got 6.8 mpg on the highway. With 197,000 miles on the motor, we thought maybe the engine or just the head gasket was blown; we got an Actron compression tester and found from 127 pounds to 150 pounds across the six cylinders. We replaced the O2 sensor and the mileage immediately jumped up to 16 mpg.  We thought, if anything, our...  We thought, if anything, our mileage should go up from putting on tires with a smaller contact patch that were slightly taller for lower highway rpms, and an axleshaft that wasn't bent. Instead, over the next 600 miles, our mileage went from a 17-18 mpg on the highway to 13 or 14 mpg. Then one day we fired it up and the accessory belt was squealing. The power steering pump had died. We mangled the pulley trying to get it off and replaced it with this one from Gen-Right Off Road since the dealer was back-ordered, and the pulley cost less than a decent puller. After we put the new pump in, our mileage was back up in the 17-18 mpg range. The failing power steering pump significantly affected our mpg.  Then, a few weeks later, we...  Then, a few weeks later, we hit a rash of warm days and started seeing the Jeep constantly running above the 210-degree dash on the temp gauge. It already had what looked to be a two or three core copper brass radiator in it, so we performed a deep flush with some Prestone radiator flush and then swapped in a 180-degree thermostat. The Jeep was then OK on the highway, but at low speeds we started having issues. Rather than try to replace the radiator, we added these hood vents from Gen-Right Off Road. We like the look of the 5.9L Grand and wanted to mimic that. It definitely helped to get the heat out and make the Jeep run cooler in traffic and on the trail, but we are waiting for some very hot days to see how it really works.  On our next wheeling trip,...  On our next wheeling trip, the taller Goodyear tires kept contacting the fender flares and knocking them off. So we decided to do some fender trimming and add TJ flares to the Jeep ("Cheap Tire Clearance," July '08). We thought that maybe by unshrouding the tires slightly, combined with the larger frontal area of the new flares, we'd lose some mileage. But, with our plus or minus 1 mpg accuracy, we were unable to see any measureable difference.
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