The 4BT is supposed to get good mileage at about 1,600-1,700 rpm, assuming the truck is not really heavy.
The engine uses .32 to.34#/hp hr of fuel. Therefore the more loaded the engine, the more fuel used. Economy will be determined by engine load, injector, combustion bowl, tier level, injection timing and other factors more so than RPM. A stock Tier 1 4BTA 130hp engine in a '48 3/4T with a Camaro T5 (v8 variety) and 3.42 rear with 31" tall tires ran great pulling trailers, etc. at 70mph all day. The 4BT would be a little light for a big bolt if you are hauling a lot unless you keep the speed down with gear ratios. For trucks 1T and larger I recommend the 6B if you are looking to get highway speeds out of the truck. You will then also need a stronger trans.
Just my 2 cents.
