why fear the RPM? I've never got that...
the OE's don't test every vehicle for mileage... the EPA uses a calculation on speed/overall gearing/RPM/ displacement and VE.
on paper the taller gear gets better mileage, but rarely in reality- especially for heavier vehicles.
mind a vacuum gauge when operating. higher signal indicates less load... less load indicates the fuel is being trimmed and the PCM/o2 is sniffing 'crossing zero' on the lambda- which is to say it is minutely adding duty cycle to the injectors and minutely pulling duty cycle from the injectors, and the engine is achieving very close to stoichiometric air to fuel ratio's- which is HELLA better for your engine, your transmission, your temperature delta's, and your fuel economy than running the same speed @ 2/3's the RPM and drawing hella less vacuum (increased load).
o/d locked means there is a mechanical connection between the engines crank and the transmission via the torque converter's lock, and no fluid coupling of the torque converter which makes for hotter transmission temperatures... when the o'd is off, the t/c doesn't lock- which allows for torque amplification to take place and less load on the engine (at the expense of higher transmission temperature).
the lower gears (higher numerically) stated above sounds counter intuitive, and there certainly is a point of diminishing returns - you'll only be able to discover (and dial in precisely what you desire by playing with axle ratio and overall tire height) what you're looking for by use of a vacuum gauge and impacting the overall ratio by playing with those two factors. you're kinda stuck with the transmission ratio's, limited in selection with t/c stall ranges, and the camshaft/valve event-displacement-VE is going to determine the RPM range best suited for your application.... Ford tried to hit it in the general area for the widest uses, but it's up to you to dial it in.