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I think diesel is going to be your best shot at hitting 20MPG with an offroad-ready rig. A guy I work with has an lifted old Land Cruiser with a Cummins 4BT swap and went from 10MPG on a good day with the gas engine to 20MPG. A Cummins B3.3, 4BT, or VW TDI would work in a Jeep. In the Suburban you could maybe do the 4BT, but you'd be best off with a pickup truck diesel.
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