In my experience, injectors/piston/compression ratio/head work/camshafts/turbo combinations that haze at idle get the best mileage on the highway when cruising rpm is 2000-2400 range and the motor is making at least 5 psi boost.
It's a well known fact that wind resistance increases exponentially as MPH increases so the faster you go, the more wind resistance losses you'll experience. However, on modified setups, it seems that 5-10 psi boost pressure is required for peak motor efficiency. Cruising at 55mph with 0 boost has much less wind resistance than say cruising at 75mph with 5 psi boost, but that 5 psi boost in some instances will add more efficiency to the motor than the lost energy due to higher wind resistance.
Case in point my 98' 12 valve:
When it had 4x.013" 155* marine injectors with 18* timing, stock turbo, stock motor, 285 tires with 3.55 gear ratio, the truck would get 17.5-18 mpg at 65 mph cruise and 2-3 psi boost.
Raising the cruising rpm to near 2000 resulted in 75 mph cruise and 5-6 psi boost and the truck would get 19.25-20 mpg all day long.
I tested at 80 mph and mileage dropped back down to the 18-18.5 range.
I tested at 55 mph and fuel mileage picked up to 18.25-18.5 range and the motor would put along at 1600 rpm or so with 0-1 psi boost pressure.
Fast Forward to today with HX35 over HT60 twins and larger injector 5x.016" 145*. Optimum fuel economy testing at 18* timing resulted in a much lower optimal cruising mph. 65 mph is now right at 19-19.25 mpg and 75+ mph has significantly dropped back down to 17.5mpg range. Cruising boost at 75 MPH is 7-10 psi with likely equal back pressure. Cruising boost at 65 MPH is 4-6 psi (With twins, cruising boost fluctuates a lot more than single stock turbo).
FYI, every injector size I've tried has shown a slightly different optimum cruising rpm/boost range. In general, the smaller the nozzle orifice size, the less boost required to get peak efficiency from the motor.
In summary, every motor has an individual efficiency curve that typically is a parabolic shaped bell curve. Every vehicle has individual wind drag curve that's somewhat exponential meaning the faster the vehicle moves, the wind drag goes up exponentially.
Optimum fuel economy is where these two curves cross. All of the rambling above is evidence that every injector/turbo/cam/timing setup is different and will reach peak efficiency at different rpm ranges.
