That turned out awesome!
I'm late to the party on this, but maybe others can benefit.
I've been a fuel mileage geek for years, so I understand where you are coming from. What I've found, is doing the math really helps a guy decide. 3 less mpg's over the life of the van = how much money? It will help you put things into perspective.
From my experience, the biggest help in offroad adventure, is ground clearance, be it 2wd or 4wd. Large diameter tires handle washboard fire roads so much better than smaller ones. Large tire diameter gives you ground clearance between the axle bits and the ground, so cushion after you air down. You add body lift, to clear those big tires and not have them rub, live with the pain in the rear of having a lifted rig (20 and 30 somethings that want 'the look', ignore that comment). From a practical perspective, it's really that simple.
What I've learned over the years on (Fuel Economy) FE commuting long distances daily (Geo Metro), and making then measuring changes:
-Tires and frontal area; rolling resistance of tires is a big deal, as is increased frontal area. In a van, you live with it.
-Engine size/efficiency; Turbo Diesel is best for FE, but live with what you bought or have, it's not worth the $$$ to throw the baby out witht he bathwater and start over, you'll likely never recover the $$$ through selling and buying something else. Again, do the math.
-Driving habits; get a scan gauge II and velcro it onto your dash (plugs into underdash OBDII port). Instant feedback that trains the driver to drive for FE, pays for itself in a year of driving
-Gearing the correct gearing for the tire diameter is important, also keep in mind the criusing speed you intend on driving. Taller is not necessarily better for FE, keeping the engine turning at the low side of the sweet spot int he torque curve, is.
-Roof racks, lights, big mirrors, aggressive looking but very functional bumpers and winches; great for utility and for getting you out of a jam, not good for FE. It's a tradeoff I'd gladly make again.
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1995 E350 7.3 Diesel, 4x4 high roof camper, UJOR 4" lift
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