How Many MPG Does A 6.6 Duramax Get? Here's What Drivers Are Saying

A perennial competitor to Ford's Power Stroke V8 and Ram's Cummins turbo diesel inline six-cylinder engines, the 6.6-liter Duramax is one of the most popular diesel powertrains for heavy-duty trucks, and has been for some time. The history of the Duramax diesel begins in 2001, when it was introduced as a replacement for the General Motors-Detroit Diesel V8 engine in vehicles like the 2001 Chevrolet Silverado 2500 and 3500 HD as well as the GMC Sierra 2500 and 3500 HD. Jointly developed with Isuzu, original versions of the V8 turbo diesel had 300 horsepower and 520 pound-feet of torque, but contemporary iterations have since upped the ante, with 2025 Chevy Silverado HD and GMC Sierra HD trucks able to reach up to 470 horsepower and 975 lb-ft of torque.

Despite its fame and reputation, however, official 6.6 Duramax mpg estimates are not publicly available from the Environmental Protection Agency (EPA). The engine is almost exclusively used in heavy-duty trucks, and the EPA's fuel economy testing doesn't cover vehicles as heavy as the Silverado and GMC Sierra. Without official test results to reference, finding reliable 6.6 Duramax mpg ratings can be tricky — which is why we've done all the legwork for you, scouring the internet for 6.6 Duramax fuel consumption figures in real-world driving conditions.

In one Reddit discussion about the fuel economy of the 2024 GMC Sierra 2500HD Denali Ultimate 6.6 Duramax, commenters wrote that they usually get between 13 and 17 mpg when city driving and 17-23 mpg on the highway. Their responses are consistent with what many drivers are saying elsewhere online about the 6.6 Duramax mpg. In a separate Reddit thread, several users mentioned getting similar fuel consumption levels in normal driving situations, but these numbers tend to dip when towing.

One owner of a third-generation 2006 LBZ – the very engine that gave the 6.6 Duramax an edge over competing diesel powerplants – noted that when unloaded, their diesel engine gets between 17 and 18 mpg of fuel economy. But when towing, those numbers drop to around 10 to 12 mpg, depending on weight and road conditions. 

Other commenters have also reported 2-4 less mpg when towing, with one Redditor writing that they get as little as 6 mpg when towing. Over at Duramax Forum, a 2019 3500HD driver mentioned achieving 8 mpg with a 28-foot enclosed trailer in tow. Along with weight, road conditions, and driving habits, larger or dual tires also tend to have effect on 6.6 Duramax mpg performance. So, you can expect to see lesser mpg overall with a dual truck or when you outfit bigger tires.