Part II: Fuel Pinching On The Rocks.
A dozen years ago, on behalf of Chevrolet/Geo, I guided two Trackers over the Rubicon Trail. Both rigs had aluminum underbelly cladding. Steve Kramer, co-owner of Calmini Products, prepped one of the vehicles to serve as a lead and assist rig. A Warn winch, 2 1/2-inch lift kit, and locking rear axle device enabled this two-door Tracker to help a virtually stone stock model through the roughest stuff.
Coming down the Sluice Box into Rubicon Springs after 18-plus hours of driving and winching, my support vehicle was nearly out of fuel. The modified lead vehicle had consumed twice as much fuel as the stock vehicle! The reason was clear enough - off-pavement on the rocks, "mileage" is not measurable. Fuel consumption is the product of 1) engine running time, 2) load, and 3) the engine's operating rpm.
Your off-road driving habits have a strong influence on the amount of fuel consumed. If you hold the engine at a higher rpm in low-range and the lower gears, economy suffers. For saving fuel off-highway, use the gear that enables adequate pulling power, reasonable compression braking and the least rpm necessary. It may be prudent to be in second or even third gear of low range, with the engine speed low, rather than in first gear with the engine revving high. Use of a higher transmission gear with light throttle adds the benefit of less torque available at the wheels - this is a plus when you want to reduce tire spin. You can always shift down a gear for better compression braking on down slopes or for rock crawling.
Engine Prepping for the Trail
For a carbureted engine, severe altitude changes can require disassembling the carburetor and making internal jet changes. Some carburetors are sensitive to altitude changes of 1,000 feet or less. In the early '90s, I performed detailed air-fuel ratio testing on various Holley 2300 carburetors fitted to Jeep 4.2L sixes. For some carburetors, the jets needed changing at altitude fluctuations as slight as 500 to 800 feet! Such an engine tuned for sea level will run very rich at high altitudes. Similarly, high altitude tuning will create a very lean burn at sea level. Lugging a rich burning engine can foul spark plugs and cause misfire. Lean burning will cause detonation (ping) or, worse yet, pre-ignition and piston damage.
These problems were partially remedied by electronic ignitions that provided higher firing voltage with wider spark plug gaps. In the '80s, clean air standards led AMC/Jeep and other manufacturers to use feedback carburetors. These carburetors had primitive means for altitude compensation. In the '80s, module-and-feedback ignition distributors achieved more complete fuel burn. The '91 Grand Wagoneer was the last carbureted Jeep engine.
Modern fuel injection readily compensates for altitude and changes in atmospheric pressure. The engine management computer constantly adjusts and stabilizes the air-fuel ratio, whether at sea level, atop the "fourteeners" in the Colorado Rockies, or inching over the Himalayas. (At cruise and light loads, the optimal or "stoichiometric" air-fuel ratio for gasoline is 14.7:1.) With the capacity to adjust spark timing electronically, EFI contributes to the efficiency and predictable behavior we enjoy in modern engines!