Gentlemen,
(I went back & reread the original post & realized I didn't answer his question.)
Dan,
Changing the fuel economy of your truck requires re-engineering. You can do an OD tranny swap (best choice); add OD like Gear Vendors, swap rear gears which will reduce towing/hauling capacities; or upgrade to modern motor, ignition, & fuel system.
(Disclaimer: some #'s will be wrong, just used for example, & some factors will be left out like weight & wind resistance.)
Whenever someone is worried about fuel mileage, we are probably talking about cruising speed. Cruising speed is all about power vs. resistance (leverage). Ask yourself how much HP/TQ is required to maintain a certain speed. Cruising speed requires only a fraction of an engines max (advertised) HP. That is why cruising RPM uses much less power than max RPM. This is why we have OD transmissions today. Engineers designed our beloved old trucks with high ratio gears (ex. 4.11:1 vs. 3.42:1) because they didn't have a lot of power. They also didn't drive very fast in 1950. I assume pulling 8,000 lbs @70mph with a 3/4ton truck wasn't even heard of in 1950. It's all about leverage. Today's vehicles have more power, which means higher speeds & towing capacities with lower RPM's.
This brings me to the engine wear issue. Less RPM means fewer times each piston goes up & down(among other things). Let's say a 235 can produce 20HP @2000RPM & that is all that's needed to propel your truck @ 60MPH. But, your 235 is spinning at 4000RPM @ 60MPH producing 150HP. That is more RPM/HP than you need, right? Then you lower the gear ratio to produce a certain speed at a certain RPM. The only problem is you lose acceleration. It's all about leverage. You have to find a balance between power(acceleration & pulling power) vs. result (RPM & economy).
Now, I know some of you are going to blast me, but it's all about helping someone maximize the potential of his truck while realizing the limitations. These are not new trucks.
