[slowmover]

If you're going to keep the grille blocked you might ought to think of adding engine monitoring (egt, etc) as things can get out of hand quickly with a diesel. Frankly, if the truck isn't being worked, then a cheaper gasser would be a better choice all around.

As to the 12V's being the best for mileage you'll get arguments (I think the 6.2 GM gutless wonder had it), and for those otherwise unfamiliar, yes, the Cummins is best since 1990, but the last "best" year was the 305 ISB in early '04. '03 & '04 CTD's are at a great price/mileage point now, in general. The 5.9 is better than the 6.7. Manual trans over automatic, 2WD over 4WD, etc.

Auto trans tend to be sloppy and the aftermarket can help when it's time to rebuild. That, and proper tires would do a lot as the truck ages.

I've measured a number of oem truck spoilers and the average low height above ground is 7-10" or so. There are some threads around here about cleaning up the underside and there might be some pieces that would work for a 4wd used off-road.

I'd say driving style is the main thing. On a 45-mph road only accelerate up to 35 mph and drift upwards slightly from there if need be. The penalty of coming to a full stop, or numerous ones, is easily offset by staying more miles on a freeway. Distance can be your friend. Trip planning is the most important. Don't turn the key without a mental picture of the route in place. Real men use maps. GPS navigation is almost worthless in this regard.

I was on one of my usual 320-mile commutes a few days ago (responsibilities in two cities) and noted to my wife that, even after over 200 miles, just 20-minutes of high idle with Max A/C lost us a full 1/2-mpg (27.5 down to just a hair under 27) off that tank filled at departure.

Look up the HP/TQ chart for your year of CTD. Note the rpm point at which the curves cross. That is almost an ideal cruise rpm. A common rail ISB is capable of .32 BSFC at 2,000-rpm, so anything under that is great.

On mine, 1,700/25 rpm translates to 58 mph. On these 740-mile roundtrips I can trip plan 24-mpg empty or loaded, rain or 100F and am always traversing one of the largest cities in North America. Highs are above 27. Travel slower than 80+% of traffic (learn to use mirrors), keep lane centered (get alignment perfected, not just acceptable), know ALL stops ahead of time (no jumping on and off roadway); in short, plan every trip.

When I am loaded or towing I plan all stops (as when I was a truck driver) for two hour intervals. So, depending on whether or not I needed fuel, these stops included highway rest areas, truck stops, restaurants, and my destination. Since even my personal rig was 63' long I often used satellite imagery (BING) or GOOGLE "Street view" to confirm ingress/egress. No backing, no unpaved lots, etc. Whatever your business, knowing exact routes to suppliers, etc from customer location ahead of time is crucial. An indecisive driver behind a diesel doesn't stand a prayer.

Once you know what is possible (and a dually 3500 4WD with auto trans is the worst specification) then spending money to save money will make future purchases sensible (thousands of miles of driving to have accurate figures). Wheel covers and fender skirts on a DRW are pointless compared to changing to SRW for reduced rolling and aero resistance (as noted above). It's ego stroking. A DRW isn't more stable than an SRW, but it does have greater payload capacity and more drive axle resistance to movement due to increased tread. True stability means the drive and steer axles are the same width. In other words, get the truck set up for the way you actually use it.

Never idle and never stop. Keep a record of elapsed times, as any average road speed above 25-mph is key. If mine is above 27 mph for engine-on time, then fuel numbers always look great. Take note of wind direction and speed. 5-8 mph tailing wind tends to offset worst of rear aero, IMO.

Consider an exhaust brake as well. Weigh the rig empty (driver, full fuel, normal tools and supplies) and then on occasion check with a load so that you know the "break even" point (1,000-lbs won't affect much, 2,100-lbs might/will?). And any lift kit or suspension/tire changes from stock are likely to backfire as far as mpg. Same with "tuners".

Your logbook needs to include:

fuel used
miles traveled
elapsed time (hourmeter)
calculations for mpg and average speed

I use 12-entries per fillup to track maintenance, etc. I like to change air/fuel filters at oil change intervals of 6-mos/6,000. Investigate fuel filtering and other upgrades. Keep a spare VP-44 on hand. Look into the dowel pin problem. Etc.

Good luck, look forward to more.