[Big Dave]

"...it would be interesting to see how the OP could improve the fuel economy without sacrificing the versatile nature of a full size SUV."

Big Dave says:
I could have sworn I outlined plans for both in the initial post.

Aero improvement on these cereal boxes is tougher than on pickups where a 'aerocap" can deliver you a solid 3 MPG.

But still I stick to my proven formula.

Diesel power
Manual transmission
Lowest (numerically) possible gearing


In the initial post I mentioned the T-56 transmission. I've been looking at its big brother - the Tremec 6060. Good for 600 ft-lb of torque. Considerably easier shifting than the ZF6-650 on my Super Duty. Where the ZF6-650 is a four speed with a granny and one overdrive, the 6060 is a four speed with two overdrives. It offers an outstanding variety of gear ratios. For a big SUV, you'd want the numerically highest low gear ratio and the 0.5:1 top overdrive. You could drop 70 MPH cruising (with stock rubber) down to 1400 RPM and gain a bunch of MPG.