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Diesel Fuel Economy Comparison- Economy Challenge

Old versus New, 2wd Versus 4WD

Jason Sands
Sep 1, 2008
Photographers: David Kennedy, Jason Sands
Photo 2/25   |   We brought Project Rust Bucket along for the ride, and had high hopes of a good finish as long as speeds were kept low.
Diesel Fuel Is Expensive. Less Than A Year Ago It Was Cheaper than gasoline, and now it is almost 30% more than unleaded. High fuel prices are something that we're going to have to come to terms with; and to help with that, we tested five vehicles on the same day, on the same route, and at the same speed. Not only would we get results, but we would get to compare old versus new, two-wheel drive vs. four-wheel drive, and emissions vs. no emissions. Older Dodge Rams ('89-'93) are notorious for being fuel mileage champs, so we picked up two for the test-our own Project Rust Bucket, which is two-wheel drive with a three-speed automatic, and Ken deVoogdt's '93 Dodge, which is a 4x4 with a five-speed manual transmission. Ken's truck is also lifted and on taller and wider tires than stock, which certainly affects mileage. For newer vehicles, we had an '07 LMM truck that was four-wheel drive, and a two-wheel-drive '07 LBZ. The LMM had EFILive, which was used to turn off the factory regeneration sequence during our drive, and the LBZ had an "economy" tune loaded because it was a Pacific Performance Engineering (PPE) shop truck. We also brought our '07 Jeep Grand Cherokee CRD along, which had the advantage of having the smallest engine, but also had a lift, different tires, all-wheel drive, and full emissions equipment. Would technology prevail? Could the lighter weight of the old trucks be an advantage? Would the Jeep take the win despite of its emissions and drivetrain limitations? We won't spoil the ending just yet, but we will tell you that you may be surprised by the results.
Photo 3/25   |   diesel Fuel Economy Challenge 1993 Dodge Ram
To start, we went over to the local Chevron station and filled up all the vehicles. There were some shenanigans, such as truck rocking and topping off (mostly by Associate Editor Jason Sands in Rust Bucket), but not too much of an effort was made to do anything sneaky. In fact, everyone was running the same tune and tire pressure as they would any other day, giving the test a "real world" flavor. From Los Angeles, the trip traveled up the 101 Freeway along the coast toward San Luis Obispo, a 181-mile drive that would test our mileage theories. We had planned to average about 65 mph, but hit some traffic that slowed us to 35-50 mph on some sections because it was Memorial Day weekend. This actually might have helped our mileage case, as our average speed ended up somewhere around 60 mph. Besides driving slow, almost everyone admitted to driving "to win it," which meant no quick accelerating and coasting down hills (generally driving like Prius owners instead of diesel truck enthusiasts). Mother Nature was determined to counter our driving techniques, however, and sent a rainstorm our way. Water on the road makes for more friction, which makes for worse mileage. Still, after about 60 miles it cleared up nicely, and we were able to enjoy the rest of the drive up the coast with clear skies.
Photo 7/25   |   David Kennedy talked strategy to everyone before we left the Chevron station. It turns out most people just planned to drive conservatively.
When we got to Oceano, California, it was off to a gas station to see how we had done. All five vehicles were topped off with surprising results. Our conservative driving habits meant that not a single vehicle was below 20 mpg! It turns out that emissions, a lift, and all-wheel drive does hurt, as the Grand Cherokee finished in last place, taking 7.75 gallons and averaging 23.35 mpg. Right in front of the Jeep and amazingly close to a tie was Ken's '93 Dodge, taking 7.74 gallons for an average of 23.38 mpg. A big jump took place when we moved on to Third Place, which is Tommy's '07 Chevy. He ran the entire event on the same tune he uses to run 12.40s at the track, and it turns out that an efficient turbo and lots of timing are good for fuel economy as well. Jumping more than a gallon ahead, Tommy's four-wheel drive D-max used 6.61 gallons for an average fuel consumption of 27.38 mpg. Finishing second with no overdrive and horrible aerodynamics was Project Rust Bucket, which was certainly helped by the slow pace. The early 12-valver managed the run with only 6.53 gallons, giving an average of 27.71 mpg. The leader of the pack for our 2008 fuel economy shootout was Joe Komaromi's '07 LBZ-engined truck. Joe had one of the lightest vehicles with the most technology, and it paid off to the tune of a miserly 6.13 gallons used for an impressive average of 29.52 mpg.
Photo 8/25   |   diesel Fuel Economy Challenge 2007 Chevy Silverado Gas Pump
So what did we learn? First and foremost, we learned that slowing down really does help, as does driving like a ninny. Technology seems to help too because the newer trucks got really good numbers. Emissions does hurt mileage considerably-the Jeep that should have gotten about 30 mpg ended up only getting 23. Finally, the old mechanical trucks still can keep up, and with an overdrive transmission and lockup converter in Rust Bucket, we think it could have won. Thanks for riding along with us on our first fuel economy challenge, and we hope to see some readers in person for next year's event!
The "Real" numbers
We Could Already Hear The Complaints About How Our Test Wasn'tRepresentative of "real" driving (speeding), so on the way back we split up and drove at our own pace. At 80-85 mph, Tommy's LMM dropped from 27 mpg to 19 mpg, while our '89 Dodge dropped to 20 mpg when we drove it home at 75 mph. Joe drove home at 70-75 mph and still got about 26 mpg with his "super economy" tune, and later admitted he had his tires at 80 psi for the test, which certainly helped.
Old Iron
On The Way Up The Coast We Saw Quite A Few Street Rods, As Well as stockers that were for sale at $6,000 each. The Model A particularly got our attention as we thought how cool would it be to putt around in an old diesel rod. Although a 4bt would be tempting, we'd opt for a Kubota tractor engine that made about 40 hp. That way we'd get a gazillion miles per gallon, and it's doubtful you'd want to go much faster than 55 mph in one of those tin cans anyhow.


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