Ram's corporate parent, Fiat Chrysler Automobiles, lent ENR a truck to test for a week, and it delivered on its fuel-sipping promise. My basic test method started by simply topping off the fuel tank at the start of the week and then going about my normal business. Errands included trips to the home center to pick up painting supplies and hauling around landscaping materials, as well as a few leisurely cruises. Seven days later, I calculated total miles driven divided by total gallons pumped into the tank during the same period.

The result was a nice surprise, with 20.5 mpg in city driving beating Ram's estimated 19 mpg with four-wheel drive and a 3.55 rear-axle ratio. The truck's computer said I had driven 19.3 mpg at an average speed of 21 miles per hour. Next time, towing will be in order to see how the engine performs under stress.

On paper, the 3.0-liter V-6 EcoDiesel—with only 240 horsepower—is not the quickest draw out there. However, torque kicks in at 420 lb-ft, enough to pull a trailer weighing up to 9,200 lb. That's not as high as some gasoline V8s but is still respectable given the savings at the pump. Unlike many V-6s, the diesel, which comes with Ram's eight-speed automatic transmission, promises a higher resale value, though its market is still untested.

The cost of efficiency is not cheap, though. EcoDiesel's starting price is $30,690, including delivery, for a Tradesman work truck with a regular cab and long bed. Most models are eligible for the option, but EcoDiesel is not available in a regular cab with short bed due to the length of its tailpipe scrubbers. Sport trims also are off the table.

Otherwise, the EcoDiesel costs $2,850 more than a comparable Ram equipped with a 5.7-liter HEMI V-8 with the eight-speed automatic transmission and $4,500 more than Ram's 3.6-liter V-6. Our test truck was loaded with Laramie Limited Edition trim and other options, such as air suspension, four-wheel drive and RamBox cargo bins, that brought the window sticker to an eye-popping $58,015.

Inside, the steering wheel is thick, making it easy to grip while wearing gloves, and the cab is one of the quietest around. However, large gaps between body panels and wobbly interior trim pieces were disappointing. Rear seats flip up easily with one hand, and you can have a flat-loading surface if you specify the fold-out storage option. I found that quite handy for loading 40-lb bags of water-softener salt when the bed was full.

Voice recognition was excellent with Ram's Uconnect technology, and I was glad to see an analog gauge for diesel exhaust fluid. Storage space in the cab is superb, though my smart phone kept falling out of its console tray. Under the rear floor mats are deep compartments, and the RamBox cargo bins built into the bed rails add even more storage. The key fob locked the doors, RamBoxes and tailgate in one fell swoop, too. A bed divider cleverly doubled as a bed extender, but the optional tonneau cover came with flimsy hardware that was tricky to install.

For construction users preferring a V-6 but needing the occasional grunt of a more powerful engine, this truck could prove more economical than a gasoline V-8 or heavy-duty diesel—especially for high-mileage fleets—though the engine's higher price tag could eat into the return on investment.

Nissan is soon to join the new diesel class with a Cummins-made 5.0-liter V-8 Titan pickup, and Chevrolet for 2015 is planning a 2.8-liter inline-four for its next-generation Colorado midsize pickup. Will there be more? It's a distinct possibility.