It is no news that diesel engines get a much better fuel mileage generally, compared to gas engines, and this is why most drivers, especially truck drivers prefer diesel engines, and also why diesel engines are in higher demand today.
The 6.6 Duramax diesel, as known, works perfectly with V8 engines and was created for higher quality and heavy-duty engines that are increasingly in vogue. It was created by a joint venture consisting of Isuzu and General Motors (GM).
However, the question is, what is the fuel mileage for a 6.6 Duramax diesel?
The fuel mileage for 6.6 Duramax diesel can get at most 15 miles per gallon (MPG) of fuel when it is on the highway. However, the average fuel mileage for 6.6 Duramax diesel is set at 14 to 15 MPG.
What is a 6.6 Duramax Diesel?
The 6.6 Duramax diesel was produced by General Motors, in partnership with Isuzu to solve problems around the efficiency and functionality of heavy-duty trucks and vehicles generally.
It is called a turbo-diesel engine for heavy-duty trucks, and it is said and proven to be the engine that has the best post-emissions among all the Duramax engines that exist at this time.
With the 6.6 Duramax diesel, there are almost zero issues around the emissions systems, as well as the injection pumps as there were with other Duramax diesel engines that came before.
The 6.6 Duramax diesel is an improvement on others, and its fuel mileage is also a large reach, compared to other diesel engines for V8.
What is the Fuel Mileage for a 6.6 Duramax Diesel?
Well, even though the generic answer to this would be that the fuel mileage for a 6.6 Duramax diesel can get up to 17 to 18 MPG, it widely varies with the type of vehicle and the way diesel is being used in the vehicle.
With different ways of driving come different fuel economies. This means that your fuel mileage is very dependent on your speed, your driving abilities as well as the amount of load or cargo you are pushing with the vehicle.
So while some engines might have a fuel mileage of up to 18 MPG, others might not even push up to 14 MPG, and still, yet, you cannot base opinions on these figures, since they vary.
This is why we visited Fuelly.com, as they have done practical research and generated concrete data to arrive at the result that answers the question about fuel mileage for 6.6 Duramax diesel.
After analyzing and averaging the data gotten from Fuelly.com, concluding that the fuel mileage for a 6.6 Duramax diesel is an average of 13 to 15 MPG, depending on all the varying factors would be pretty reliable.
Also, you can watch the video below made by the Daily Motor YouTube channel where he did a real world MPG test on a 6.6 Duramax Diesel engine using his 2024 Chevrolet Silverado 2500 Duramax Diesel.
What Year Did Duramax Get the Best Fuel Mileage?
The year that Duramax got its best fuel or gas mileage was 2021, as the 6.6 Duramax diesel, in the Sierra 2500HD series more specifically, gets as high as 17 to 19 MPG, especially when it is on the highway. This will however get better with the new models.
Another real life miles per galon test made by Daily Motor YouTube channel  on a 2022 GMC Sierra 2500 Duramax Diesel turned out not so bad. You can watch the video below for better understanding.
In abnormal situations, however, a Duramax diesel can go as high as 22 MPG, or even higher, as a lot of people have given an account of how their engine ran at 22 miles per gallon.
At the end of the day, it depends on how you are driving, at what speed you are driving, the style with which you are driving, whether the road is smooth, or you are unavoidably driving rough, and also what you are carrying or pulling with the engine.
What is the Fuel Mileage for 2020 6.6 Duramax Diesel?
The average rating of 2020 is 6.6 Duramax diesel has been put at 19.6 miles per gallon. This is after a lot of research and testing. However, the truth is, the 19.6 miles per gallon rating is a reach, and it varies on a lot of things, as earlier said.
A vehicle carrying too much load will not reach the rating that has been measured, the same way when drivers drive in a kind of way, too rough, or on a rough road, the miles per gallon will also vary.
On a highway, the 2020 Chevy Silverado 2500HD diesel has been estimated to go as far as 33 miles per gallon as well, when conditions have been tweaked, such as turning off the AC, driving on a new fuel filter, upgrading the intake of air in the vehicle, and other upgrades.
Conclusion
Although the fuel mileage for a 6.6 Duramax diesel engine is said to be an average of 15, in the real world scenario, it is vastly a different case, as a lot of factors can influence the mileage of a 6.6 Duramax diesel.
To recap, the influences that can either heighten or reduce the fuel mileage of a Duramax diesel include; the speed at which you are driving, the style of driving; whether rough or smooth, the cargo you are carrying or pulling, as well as the type of upgrades, or degrades you have effected in your 6.6 Duramax diesel.