Increased fuel efficiency has apparently struck a chord with full-size pickup-truck buyers: Chrysler said yesterday it would double production of its 2015 Ram 1500 EcoDiesel in response to soaring demand.

Ram announced that it will now build the diesel powertrain mix into 20 percent of total Ram 1500 volume--fully twice the number the company originally expected to produce.

ALSO SEE: 2014 Ram 1500 EcoDiesel: Quick First Drive Report

Demand for the diesel half-ton truck has been robust from the start.

When it first started taking orders earlier this year, Ram received 8,000 requests in just three days, completely selling out the initial allocation for the 1500 EcoDiesel.

2015 Ram 1500 EcoDiesel

The only diesel half-ton pickup currently available, the Ram 1500 EcoDiesel boasts an EPA fuel-economy rating of 23 mpg combined (20 mpg city, 28 mpg highway) for rear-wheel drive models.

Four-wheel drive models are rated at 22 mpg combined (19 mpg city, 27 mpg highway).

Some drivers are reportedly exceeding those mileage ratings in the real world, with anecdotal mileage estimates in the low-30 mpg range.

DON'T MISS: 2014 Ram 1500 EcoDiesel Pickup Fuel Economy: 23 MPG Combined

This isn't unknown for diesel vehicles; Volkswagen Jetta TDI drivers have bragged about exceeding the mileage figures on their window stickers for years.

However, it's fairly unusual for drivers of full-size pickup trucks to brag about their mileage--especially fuel economy above 20 miles per gallon. For standard pickups, 30 mpg was seen as an unattainable dream just a couple of years ago.

2014 Jeep Grand Cherokee EcoDiesel, Catskill Mountains, NY, Jan 2014

The 3.0-liter EcoDiesel V-6 engine is also used in the Jeep Grand Cherokee EcoDiesel.

It's manufactured by VM Motori, a division of the Fiat Powertrain unit of Ram's parent company Fiat Chrysler Automobiles.

The engine produces 240 horsepower and 420 pound-feet of torque; the sole available transmission is an eight-speed automatic.

Like the rest of the Ram 1500 truck range, the EcoDiesel is equipped with other fuel-saving features, including active grille shutters, aerodynamic under-body panels, and an engine stop-start system.

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