To get more FE...drive faster?

[Sludgy]

You didn't say anything about the automatic tanny: Does it have a lock up torque converter? If it doesn't, there could be a lot of torque converter slip at lower speeds, and this would torpedo your mileage at low speeds.

A similar thing happens in my F350. The torque converter will simply not lock up under about 45 mph, no matter how much I baby the throttle.

I've been looking for a chip to reprogram the tranny, but no luck so far.