Easiest thing to do would probably be to put a lower ratio differential in. It would probably make at least a couple of MPG difference, I would imagine. Especially since the car is geared with such a high engine speed on the highway. The 722.4 tranny also has a 1:1 4th gear ratio and no torque converter lockup clutch, so that doesn't help a whole lot either.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record