Your questions make perfect sense and are welcome! My opening volley on this topic reflects my perspective, and I'll elaborate in more detail...
Best axle gear ratio is a somewhat loaded question and does consider tire diameter. I wrote magazine Q&A columns for fifteen years in the 4WD Jeep, 4x4 truck and muscle car fields. Gearing was a constant question with tangible solutions. In the late '80s, I got a nifty calculator program from Wolverine, intended for camshaft selection, tire sizing and the right gearing choice. Though DOS based and no longer accessible (thanks to Microsoft 7 and up), that program provided a great calculator utility.
Fortunately, there are similar calculators for axle gearing and tire diameter now online. Here is one downloadable software calculator that you can trust. It's directly from Cummins and includes your '06 Cummins 5.9L ISB Dodge Ram 4x4 Mega Cab—directly from the source: http://www.powerspec...home/index.html. You'll find a great calculator for Cummins' commercial diesel engines. Note: I use the earlier version 4.2.4 software for the 5.9L ISB engine; the latest version only covers the 6.7L ISB, although this engine is similar in most ways for this calculation.
I use this program, and it's very cool, addressing tire diameter, axle gearing (include the 0.69 ratio for overdrive on the 48RE) and engine rpm. Cummins has a recommendation for optimal commercial performance, which is actually close to the weight and load factors you and I anticipate: occasional towing, hefty accessorizing, the lift (which makes our trucks like pushing a billboard down the road) and both town and, primarily in my case, highway use.
There's one caveat when using the PowerSpec program: The program aims at commercial haulers. Cummins wants to see a higher rpm (2100-2400) than I prefer. My approach for maximum fuel efficiency with a modified truck weight similar to ours (9000-9300 pounds curb weight) and the 5.9L H.O. ISB engine is in the neighborhood of 1900-2000 rpm at highway cruise. I see a prompt and notable drop in fuel mileage for engine speeds over 2000 rpm—although power remains great between fuel stops!
For gasoline powered trucks or a trail Jeep, here's a quick reference chart that you'll find useful: http://www.jeep4x4ce...om/calculators/. The chart's baseline is 65 mph with a 1:1 high gear ratio. You need to factor the overdrive ratio into the final engine speeds at cruise. Multiply the rpm times 0.069 for your 48RE transmission. We have a significant 31% overdrive.
There are other calculators online that allow for plugging in the overdrive ratio variable (0.69, 0.75, 0.85, etc.). Here is one that offers space for the overdrive ratio in the equation: http://www.4lo.com/4LoCalc.htm.
As for picking between 4.10, 4.56 and 4.88 gears (the only ring-and-pinion options for the AAM axles) in your specific truck, the answer is subjective. I'll tackle the question, though, and will share what I believe each of these ratios will deliver with your 37" tires:
1) 4.10:1 would nearly restore your gearing to the OEM level with stock diameter tires; still some overdriving effect, more like OEM tires with 3.55 gears instead of 3.73:1. I would definitely not use this gearing for trailer pulling. Town traffic would be sluggish, too.
2) 4.56:1 would be quite livable, all around. Acceleration would be slightly better than OEM tires and gearing at the OEM curb weight and normal cab height. This is my gearing now for 35" tires, and I know that a change to 37" would be feasible. I'm spinning the engine close to 2,000 rpm at 65 mph...To drive at 75, like you want, the Cummins calculator actually thinks the engine should spin faster—I think it would be right on for performance and reasonable fuel efficiency at this load. (I'm figuring 37" tires at 560 revs per mile. Is this correct for your tires? Confirm for calculations.) 560 revs per mile and 4.56 gears in overdrive cruise mode would put your engine at 2202 rpm for 75 mph and 1909 rpm at 65 mph. This would deliver peak fuel efficiency at 65 mph and decent performance at 75 mph, a satisfying all around choice for your truck with the 5.9L ISB engine.
3) 4.88:1 would be okay if you trailer all the time and would like to hold speed at 65-70 mph while towing. (You could push to 70 mph if fuel costs do not sway your thinking.) Acceleration in stop-and-go would be impressive, the load on the engine and 48RE transmission would be less. Extra piston travel per mile could reduce lifespan of the 5.9L engine; however, the reduced load would likely offset this...If I had a 9-horse trailer, this would be my gearing, holding the truck to 65 mph and keeping the engine and horses happy!
As for target mph on the highway, that's a subjective question, too. You have a plan for 70-75 mph at cruise, and with a trailer, that would keep the engine at Cummins' recommended 2100-2400 rpm range in overdrive. While I repeatedly emphasize the 1600-1900 rpm "sweet spot" for fuel efficiency, there is the realistic lugging factor that places the engine under stress below 1900 rpm when toting severe loads.
So, I think you'll be happiest with 4.56:1 gearing at 37" diameter tires. I can feel with our truck that it's doing just what I wanted: Delivering trailer pulling torque, adequate horsepower and peak fuel efficiency at 65 mph. 4.10:1 would have restored the tire/gearing to stock, but as I've noted, this truck is way too heavy and tall at curb (unloaded with fuel in auxiliary tank) to survive on stock equivalent gearing. I've equated my modified truck to pulling a tent trailer—all of the time!
As for highway versus town driving, you need to consider both. Town driving is getting the mass rolling. Highway is keeping that mass rolling. Both impact fuel efficiency and loads on the engine. Again, the 4.56:1 will enable town driving without taxing the powertrain much. 4.88:1 would make town driving easier, especially when moving a big trailer from a dead stop; however, the highway cruise engine speeds you plan would make 4.88:1 wasteful on fuel. (2,357 rpm at 75 mph in overdrive is near peak rpm for Cummins commercial use recommendations.)
You're right about dyne tests to confirm power curves on a dramatically altered engine with camshaft modifications, fuel timing changes, added turbo boost and other alterations. Let me start by saying "chips" will not dramatically alter the torque rise on a diesel engine. Tuning measures will unleash suppressed power, but the curve shape will be similar.
Camshaft, compression or turbo mods are another story, as changes here can move the power curve around. Although everyone seems horsepower fixated, diesel power is all about the quick torque rise and peak at a lower rpm. I believe the Cummins ISB engine has the edge here, a lower rpm, traditional diesel design suited for medium duty truck use and patterned after commercial truck and off-highway equipment performance.
That said, drive your Cummins diesel accordingly. I have "redlined" our truck's engine to its advertised 3,400 rpm peak on less than a half-dozen occasions in 121K miles. Redline is pointless when the engine's torque peak is at 1,600 rpm. 2,400 rpm should be a sensible peak, maybe 2,800 rpm on a lengthy grade with our trailer in tow—and certainly not for a sustained period.
Note: Gale Banks and I met after the Off-Road Expo in 2011. I visited Banks Power at Azusa, and we talked. Gale is a Duramax diesel dealer and strong advocate, and his descriptive for the Cummins ISB engine was, "We blow the cylinder head off the block on those engines!" They do when drag racing with engines built for extreme output and competition—or even when running these engines "to destruction", presumably at redline for sustained periods. I have known Gale for thirty years, and he is expert at high performance and race engineering. By contrast, I served an apprenticeship with the Operating Engineers Union in the mid-'seventies, and we worked large highway construction jobs. If we ran a 1693 Cat inline six off-highway equipment engine much over 1,500 rpm, we were in jeopardy of losing our job. These engines peaked horsepower below 2000 rpm and peaked their torque just off-idle!
Trust this helps. It's really not that complicated once you establish a firm goal. Even with the four-speed automatic, I've gotten as good or better fuel efficiency with the 48RE as others with the NV5600 6-speed manual overdrive transmission. We get the added advantage of a torque multiplying converter, too!
I can assure owners with the manual transmission, Cummins engine and the right gearing that 22-25 mpg fuel efficiency is very attainable on the highway under light loads and at normal cruising speeds...It all comes down to driving technique. Running empty, my upshift points would be between 1,400 and 1,600 rpm per gear.
Trust this helps and thanks for posing these questions!