Drove my truck from Montana to North Carolina for about a month. Took 3 days of nothing but driving. Though I wasn't towing anything I averaged 17.18 mpg. With the stock 25 galon tank. Think I was going around 480 miles on a tank. The dash told me I was at 20.15. If I had the cruise set at 75 mph, it would drink the diesel as fast. Anything above that and the needle would sink quick. I was think on the way there, would putting in lower gears or going from a 5 speed auto to a 6 speed auto, should lower the rpm's a bit to keep the engine from not working as hard.

Cheapest route would be a taller tire, at least then if they dont work out you can replace them with a different size. Drop you speed down for better fuel milage or keep the tach below 2k. 2K seems to be the magic number even for my LBZ's, anything above that and start sucking fuel.