Almost since the order books first opened, consumer interest in the Ram 1500 EcoDiesel full-size pickup truck has seemed strong.

The initial order allocation was filled in just three days.

Fiat Chrysler Automobiles (FCA) announced last September that it would increase EcoDiesel engine production, aiming to have the diesel account for 20 percent of Ram 1500 truck volume.

MORE: 2015 Ram 1500 EcoDiesel Production To Double: 1 In 5 Trucks To Get Diesel (Oct 2014)

Yet while its factories are apparently installing more 3.0-liter V-6 diesel engines, sales of the EcoDiesel model are lagging, according to trade publication Wards Auto.

Through June, FCA claims diesels made up 18 percent of Ram 1500 trucks shipped from its plants, but only 12 percent of the models sold at dealers.

Retail sales will fall into line with the original 20-percent goal "once we can reach target stock levels for the dealers," Ram spokesman Dave Elshoff told Wards.

2015 Ram 1500 EcoDiesel

2015 Ram 1500 EcoDiesel

The discrepancy between production and sales figures seems to indicate that Ram is building more EcoDiesels than it is selling.

Registration data for vehicles purchased through April was even lower than FCA's sales estimate.

It showed less than 9 percent of buyers were choosing the diesel over gasoline models.

DON'T MISS: 2014 Ram 1500 EcoDiesel: Quick First Drive Report (May 2014)

At the same time, though, many dealers reportedly have low stocks of the EcoDiesel.

At one Michigan dealer, just five of 34 Ram 1500s in stock had the diesel engine. The sales manager said it was difficult for dealers to get their hands on EcoDiesel models.

Ram offers two gasoline engines in its half-ton truck--a 3.6-liter V-6 and a 5.7-liter Hemi V-8.

2015 Ram 1500 EcoDiesel HFE

2015 Ram 1500 EcoDiesel HFE

While the EcoDiesel boasts the best fuel economy of the lineup, its appeal may be limited to certain customers.

The price premium of almost $5,000 compared to the base gasoline V-6 can make the diesel seem less attractive, especially to customers looking to lease.

ALSO SEE: Ram 1500 Diesel Pickup Fuel Economy: Drivers Beat Mileage Ratings In Real-World Use (Aug 2014)

But those who intend to buy can take better advantage of the EcoDiesel V-6's superior fuel economy, by covering more miles and saving more fuel in the process.

Now that average national gasoline and diesel prices have reached near parity, the diesel should look more attractive than ever.

Whether it's production bottlenecks or lack of demand, though, the only half-ton diesel pickup currently on sale appears to be a rare commodity.


Follow GreenCarReports on Facebook, Twitter, and Google+.