The Environmental Protection Agency has yet to release its official highway, city and combined fuel economy ratings for the 2020 GMC Sierra 2500HD with the 6.6-liter L5P Duramax diesel V8 engine, but a recent road test conducted by Cars.com may give us a rough idea of what to expect.
The publication put its 2020 Sierra HD test vehicle on a 208.1-mile loop in Michigan, taking the AT4-equipped truck on both city streets and out on the highway. It was a near-perfect day for this test, with the weather perfectly clear and sunny and the ambient air temperature sitting at a crisp 60 degrees. The truck also stayed in two-wheel drive mode for the entire duration of the experiment, speeds were kept to within 5 mph of the posted limit, and acceleration and braking events were carried out in a smooth and consistent manner, Cars.com says.
While the trip computer in the truck showed an average consumption rating of 19.6 mpg, the truck consumed 10.453 gallons of fuel over the 208.1-mile trip, placing its real-world fuel economy at 19.91 mpg. A Duramax-powered 2020 Sierra 2500HD without the AT4 package may be able to achieve even greater fuel economy, as the trim comes with off-road tires that may increase the truck’s rolling resistance as opposed to regular tires. In addition to the off-road tires, the AT4 package also adds underbody skid plates, Rancho off-road shocks, an Eaton locking rear differential and an Off Road setting for the traction control, among other small items.
If you need a refresher, the 6.6-liter L5P Duramax diesel V8 is rated at 445 horsepower and a meaty 910 pound-feet of torque. We imagine the Allison-branded 10-speed automatic, which is the only available transmission on Sierra HD models, massively helps with the truck’s overall fuel economy.
When we tested the 2020 GMC Sierra AT4 2500HD earlier this year, we said we were “pleased by the Duramax engine’s power delivery, smooth and free of unwanted turbo lag.” We also noted that the Allison-branded ten-speed automatic transmission was also a gem as it always seemed to find the proper gear and downshifted smoothly when applying throttle.”