everyone is afraid of engine RPM, and equate higher RPM's with more fuel consumption... which is true... but not exactly true.
you fellers are talking about 4th and 5th t/c lockup, and the schedule for shifting- which is related directly to engine speed, and while speaking engine speed through known gear reductions (trans and axle ratio), it is directly related to speed... so, we say a transmission shifts at a given speed, when actually, it shifts at a certain engine speed and load ratio...... if the load is less, it shifts sooner- if you bury the throttle in any given gear the pcm tells the tcm to drop a gear to lever the required torque until it lessens engine load..
if you can follow all of that through my terrible attempt at communicating it, then you can follow this, too:
if an engine is loaded at 35% of it's capacity to maintain it's engine speed, and it decreases it's leverage by shifting to a higher gear- in order for that engine to maintain that engine speed using the higher gear (5th, t/c locked, let's say) it HAS to increase the engine load... so... for speaking terms, that engine load effort to maintain engine speed increases from 35% to 50%... you're running a gear higher, turning less RPM's to maintain the same forward speed as 4th gear provided, but your engine load increased- so... you are burning about the same amount at best, or even MORE fuel...
the 5th gear shift is a misnomer on either side of the scheduled shift, and possibly misdirection.. yeah, you're turning less RPM's, but you are likely burning just as much fuel... at the speeds selected for programming, it is the hinge that usually differentiates drivers and not trucks/rigs...
T/C lockup is another thing altogether- and the drop in RPM's is absolutely a red herring insofar as fuel consumption is concerned- it, imHo, should not be considered AT ALL when you're trying to drive economically.. the function of the locking device is to match engine RPM's to input shaft of the transmission's RPM while locked... it is a device to decrease what a lot of folks call 'torque amplification', but what is better described as 'torque INERTIA amplification'... when the t/c locks, the rpms of the crank match the rpm's of the input shaft, which decreases the motion and lessens the HEAT a transmission generates- that is it's purpose, make no mistake... it is a very nice little thing to protect the transmission when the engine can produce enough torque to carry the entire brunt of the truck+load. When the engine is stressed to maintain that comfortably, the computer unlocks the t/c and uses that 'torque inertia amplification' to it's advantage, and at the cost of the transmission generating considerably more heat.
you guys are speaking of engine speeds around 48~54mph, where the H&S schedules (canned) are set (again, it's easiest to describe it in forward speed, but it's actually an engine speed expressed in RPM compared to an engine load calculation- but also understand, those two items are mathematically bound- you give me overall drive gear ratio, engine speed, and if your trans is not slipping, I can give you forward speed, or give me any of those two and I can tell you the third with a certainty.. if you give me all three and they don't jive, I can tell yuo how bad your transmission is slipping) the differences in gear selection, t/c early lock or not, need to be tailored to the individual driver and their needs... you MAY find that locking later, in your case, saves fuel.. you may find that your engine produces plenty of torque for your average load, and you can lock earlier...
don't be afraid of the RPM's, though, and equate them to excess fuel consumption.. they are only cousins; they are not brothers.