Thanks for airing these questions, Megatron! Given your truck's weight and lift, as we have discussed, the rpm at 65 mph is too low—but only because of the truck's modified curb weight and wind resistance. I'm not double-speaking, you're at an optimal rpm at 65 mph for extraordinary fuel efficiency with a stone stock '06 Cummins Ram 4x4.
Proof: Before I horsed around with 1200-plus pounds of upgrades and a 4-inch lift, in the summer of 2011 I drove to Portland, Oregon from the Reno, Nevada area for the launch of the Jeep JK Wrangler 2012 model (new Jeep Wrangler 3.6L Pentastar V-6 and other changes—see the magazine's JK Wrangler HD video coverage). I consciously held to 65-69 mph with stock 3.73 gears and tires just under 32" diameter. The truck had its stock curb weight and height (approximately 7,800# unloaded).
Result: I nailed 25 mpg fuel economy from Reno to the mid-Willamette Valley, Oregon! Best mileage on a long trip ever, including climbs over the Siskiyou Passes and the numerous secondary highway grades between Reno and the I-5 access from Highway 89. This was not the "Plains" and surely not one-sided in terms of grades.
That said, if I attempted the 1,600-1,750 rpm under the truck's current curb weight and fully fueled load, worse yet when pulling a trailer, I would expect sluggish performance and mileage. Our Ram 3500 has stone stock tuning to this day, no exhaust upgrades, chips or modifications whatsoever. I drove this rpm range to achieve this exceptional mileage in stone stock form; however, the truck would suffer with that gearing and current 35" tires.
Today, cruising at 1,900-2,000 rpm peak, I can coax 21-23 from the truck with the 9,100-plus pound rolling package, unloaded and fully fueled, driving uniform gradient highways...Managed 23 mpg down the I-5 from Sacramento to the 41 Junction en route to Advance Adapters at Paso Robles in January. Held speed to an agonizing 65 mph to achieve this, got passed like I was standing still by the traffic! Pushed to 68-70 and paid for it immediately: 19-20 mpg with 35" tires, 0.69 overdrive and the 4.56:1 gears.
Last year's trip across northern Nevada's I-80, through Elko to Salt Lake City and on to Moab, I did manage 22-23 mpg at 65 mph on I-80. Frankly, the Bonneville Salt Flat and Great Salt Lake Basin is tortuous at this speed, although I did have plenty of big truck company in the slow lane. This is the price for maximum fuel efficiency. Diesel engines are rpm and load sensitive! For that matter, gasoline engines fare as bad or worse under load.
We really enjoy the truck and its "look". (Objectively, folks at trade shows and the Moab Jeep Safari rubberneck to see this Ram 3500 Quad Cab, as I'm sure they do your Mega Cab!) To a degree, I'm willing to "pay the price". However, my days of 70-75 mph trailer-toting with the XJ Cherokee on board en route to Moab, expecting 17-18 mpg, are over. I'll still expect 15-16 mpg with a trailer in tow, fully loaded, but only after my change to 4.56:1 gears and "governing" speed to a maximum 65 mph when towing. Wind is a much larger factor with the 4" lift and 35" tires, an obvious fuel mileage variable...
As a footnote to the trailer toting mileage, I lugged a toy hauler to Johnson Valley for the 2012 King of the Hammers race week coverage. I still had the OEM 3.73 gearing (like you do) and did respect California's strictly enforced 55 mph speed limit for trucks and trailers, holding speed to a limp along 58-59 mph peak. I thought the mileage would be great. Wrong—mileage fell to 12-13 mpg, the worst ever for this truck's trailer pulling and a 7,500# (loaded) trailer. It's all about the right gearing and keeping that diesel happy under load!
This raises a point we haven't touched yet. You're weighing the 4.56 versus 4.88 gearing choice for your 37" tires. How about this idea: Install a Gear Vendors overdrive behind the transfer case! The auxiliary overdrive only works in 2WD High range, but you could run 4.88s or, better yet, 5.13 replacement gears in the AAM 11.5 and 9.25 inch axles with the use of the Gear Vendors overdrive when practical—including split shifting on grades with a trailer load. See the interesting torque gains described at the Gear Vendor's website. I've linked directly to the Dodge Ram section.
Gear Vendors is a 0.78 ratio or 22% overdrive, which could only be used minimally in tandem with your 0.69 overdrive gear. Final drive equivalent in overdrive/overdrive would be 5.13 x .69 x .78 = 2.76:1. This compares to 4.88 x .69 = 3.36:1 [48RE overdrive without Gear Vendors OD]; or 5.13 x .69 [48RE overdrive without Gear Vendors OD] = 3.54. Your OEM gearing factors currently as: 3.73 x .69 = 2.57. Of course, we haven't targeted engine rpm and your tire diameter yet...We can play with the gearing equation and see if this makes sense.
Cost aside, the Gear Vendors overdrive is a consideration if you're on the fence here. ..I've got a one-piece driveline at 140.5" wheelbase, you're longer based with a two-piece driveline. Plenty of room for a Gear Vendors unit in either case. For my truck with its current 4.56:1 gearing and 35" tires, I'd have gains like 1) the double overdrives when running empty at cruise or 2) when pulling a hefty load, I could use the Gear Vendors overdrive with the 48RE in 3rd gear (22% versus 31% overdrive, a 9% reduction advantage). You haven't changed gears yet, so there are more options to consider.
I like your seasonal fuel efficiency question! Like the intercooler, winter cold air creates denser oxygen content, which increases power. If your climate is humid in the summer, you may get an additional intake air cooling effect and, therefore, denser oxygen content in the intake stream. A denser charge means improved combustion and, at least theoretically, better fuel efficiency if you can keep your foot out of the more responsive throttle!
Another factor to weigh is the OEM or aftermarket computer "chip" tuning that takes advantage of a denser IAT reading by changing injector pulse width. This could also work another way: When winter intake air is cold and dense, the injectors flow more fuel to create the right air/fuel ratio. The result is more fuel consumption and peak throttle response.
I believe the winter fuel efficiency loss is also due to lubricant viscosity and cold pour, especially in the axles. (Transfer case lube and ATF are now largely synthetic and wider-range viscosity, typically with a lower weight ceiling.) It's no small fact that OEMs now use lighter, multi-viscosity lubricants for CAFE ratings. The AAM axles call for 75W-90 weight lube. My choice of running 75W-140 Mopar gear lube for severe duty use may, in fact, create a loss in fuel efficiency—especially in the winter.
Don't get carried away with this factor, though, unless we're including engine oil viscosity. See this technical paper for interesting details on fuel efficiency as it relates to motor oil viscosity: http://www.institute...er/FEIPaper.PDF. I'm running Mopar 15W-40 year round with use of a winter block heater to avoid startup damage. Frankly, this could be a source of higher fuel use in winter. The 5.9L Cummins engine can drop from 195-degrees F to the bottom of the temp gauge during a 30-minute groceries stop!
Another factor, perhaps significant in our case with oversized tires, is rolling resistance. In winter, the advertised or usual static cold tire pressures can create drag. On frigid highways in chilling air, the tires do not reach the predicted temperatures for pressure calculations. In the summer, there is less rolling resistance as inflation pressures rise due to ambient and road surface temperature increases. Sadly, tire pressure is fixed (unless you have a Humvee and can air and deflate on the fly!). You cannot inflate tires to the summer over-the-road pressures for winter use—the tires would be deformed and wear quickly on their tread centers from "over-inflation". They would also be a terrible hazard with minimal road surface contact!
As for engine operating temperatures, and this is a factor, it does take "forever" for the Cummins engine to warm when ambient temp is sub-zero to 40-degrees F. This is not a coolant thermostat issue, although 18-wheeler shutter-stats like you hint would be helpful. The Cummins ISB engine has a huge cooling capacity! Forget letting the engine idle to warm up, it will take forever to reach operating temperature.
On that note, I'll emphasis that I do use a block heater all winter. The truck is not a daily driver, but on any day with a planned run, I plug in the block heater the night before. Under a carport with a Battery Tender hooked up whenever the truck parks, let's say it is a 10-degree F morning: The engine coolant's startup temperature will be around 120-130 degrees F with the block heater.
Warmer weather (a whopping 35-degree F, let's say), start-up temperature will be around 140-degrees F. That noted, I start the engine and always wait for the oil to circulate sufficiently before rolling. It will sometimes take from Fernley (home base) to four miles West on I-80 before the engine reaches thermostat temperature. During this entire time, I hold engine speed to no more than 1,500 rpm under light throttle load. I expect this diesel engine to last for 500,000 miles, enough to bury the initial cost difference over a gasoline engine—and also to save a small fortune through fuel mileage gains over that number of miles.
So, summer does bring better fuel efficiency. In my case, I've also switched to GDiesel, a natural gas altered fuel. (See my coverage of GDiesel in an article and HD video interview at the magazine.) By breaking down long chain molecules to more combustible form, this fuel does deliver mileage gains. Even better, I do not engulf other motorists in soot when I launch into the throttle during passing.
Yes, my truck's exhaust, even without the later catalytic converter and exhaust stream additives, is clean, without visible smoke—black or white. While some clean-up can be produced through ECM programming, that's only to a degree. I use GDiesel and avoid mixing it with conventional low-sulfur fuel. (GDiesel is actually the very same low-sulfur base fuel, reprocessed with a patented method. You can mix the two in a pinch.) Fortunately, I can make it to Moab from Fernley and back home on one fill of the main tank plus the 75 gallon auxiliary Transfer Flow tank in the bed.
Thanks for the questions, Megatron...Others are welcome to jump into the discussion and share their experiences!