In the 1960s, when performance was king, it wasn’t unusual for dealerships to take stock muscle cars and customize them to create highly desirable street machines that are avidly sought after by collectors today.

Today, a Cumberland, Tennessee, International dealership is using the same concept to provide high-performance trucks to its customers. But the emphasis today isn’t on horsepower: It’s on fuel economy.

We first featured Cumberland International's C10 project in a June 2016 cover story on breaking the 10-mpg barrier. That truck was based on an International ProStar.

The second phase of Cumberland International’s C10 truck project is a specially tweaked International LT tractor optimized to consistently deliver fleets 10 mpg in real-world trucking operations. The truck is the brainchild of Matt Smart, fleet sales director, and fleet sales account manager Patrick Mendenhall, who wanted to “think outside the box” to show customers that International was “making a great product again” while also demonstrating that a 10-plus-mpg truck is achievable to operate in day-to-day fleet operations.

Instead of waiting for a truck manufacturer's “Super Truck” or “Truck of the Future” to roll around, Mendenhall explained, the folks at Cumberland took the project into their own hands and spent most of 2013 building what the dealership calls the RX-C10, with a downsped Cummins ISX powertrain, advanced safety systems and aerodynamics.

Mendenhall said he and Smart were designated Cumberland’s “Fleet Team” by Cumberland dealer principle Terry Minor and began gathering information from resources and vendors to conceptualize the most aerodynamic truck possible.

“With ever-changing technology, this was no easy task," Mendenhall said. “Thankfully, many vendors were excited to help in any way possible to get this project started.” However, improvements the dealership established on for the final version of their fuel efficient tractor are proprietary to protect the customers who have purchased a version of the C10, he explained.

The first demonstration truck was delivered to Cumberland in January 2014, with South East Carriers located in Lawrenceburg, Tennessee, having the honor of taking it out on its inaugural run. A key concept of the C10 initiative, Mendenhall says, is to let Cumberland fleet customers test the truck for themselves in their own, real-world, every day, working conditions, to see for themselves how it performs.

The initial South East Carriers run, in January 2014, turned a promising 8.2 mpg – although Mendenhall says the route and application were particularly challenging. Since then, the C10 demonstrator model has transitioned from the original ProStar base model to an updated version based on the LT Series powered by the new Cummins X15 engine.

All told, both trucks have completed almost 20 different customer demonstration runs, with overall fuel economy of 8.11 mpg on the original demo truck (RX C10) and averaging over 9.2 mpg on the second demo truck (NextGen C10) through four test runs. The highest number so far was an impressive 9.9 mpg overall achieved by a local Tennessee fleet. Mendenhall said no two test runs are identical and the truck experiences extreme variations in driver habits, traffic, loads, wind conditions and weather each time it goes out. Customers tend to dictate the length of the runs, he added, with some keeping the truck for only a week at a time, and others keeping it for up to three months.

The C10 routinely breaks the 10 mpg mark in top gear highway cruising, Mendenhall noted. And he says the truck has proven to be invaluable to the dealership as it works to provide the most-fuel efficient truck possible to its customers.

And it’s an approach that is working, too. Mendenhall says that to date, Cumberland has sold more than 1,000 C10-spec’d International ProStar and LT tractors, and he expects orders to continue to ramp up as word about the truck’s performance spreads.

You can follow the progress of the C10 truck on www.cumberlandc10.com.

Updated 1:00 EDT June 19 to clarify differences between the first- and second-generation C10.