Why Is My Duramax Getting Bad Fuel Mileage? – Maximize Fuel Mileage of Your Vehicle

I have seen a few people talking, “Why is my Duramax getting bad fuel mileage?” Typically, Duramax engines are great for fuel economy. But often, some things reduce efficiency. In this blog, I will discuss all the reasons for the poor fuel mileage of Duramax engines.

Fuel mileage or economy is the distance a vehicle travels per fuel consumption unit. People measure it in miles per gallon (MPG). Thus, everyone values it as it directly impacts the cost of operation. In the same way, the owners of vehicles with Duramax engines often reconsider the MPG when it goes down.

Briefly, there are a few common reasons to get bad fuel mileage. They can be incorrect tire pressure, worn tire tread, defective air filtration system, and low fuel quality. But you can solve these issues quickly by following some instructions. Donot worry. I will also explain how to maximize Duramax fuel mileage.

Duramax Engines & Fuel Mileage – What You Should Know

Duramax engines from General Motors and Isuzu are famous for their robust performance and efficiency. You can find them in different models. Also, these models have variations of horsepower (HP), torque (lb. ft.), and fuel economy ratings (MPG) (1,2).

The average fuel economy you get using a 6.6L Duramax engine is 15-20 MPG when you drive on city roads. But it increases for highway driving (20-25 MPG) (3). It offers 445 HP and 910 lb.-ft. of torque (4). However, many drivers have achieved an average of 18-20 MPG combined city and highway driving with a 6.6 Duramax engine (5).

Alternatively, a 3.0L model will provide 277 HP and 460 lb-ft of torque (6). In this model, the MPG is higher than the 6.6L model engine. This engine has 23 MPG for city and 33 for highway driving fuel economy rating in a 2-wheel-drive system. Besides, the 4-wheel-drive system gets 23 MPG for city and 29 for highway driving. (7,8).

So, you can see that things are different in various models. Some drivers have reported even higher fuel efficiency. Some have reached the mid-20s or even low 30s under optimal conditions (9).

However, the problem is that this MPG goes slightly sinister over time due to several reasons.

Many reports are even there regarding this issue. For example, an owner of a truck with 2021 Duramax AT4 6.6L L5P

Duramax user reported, “I used to get 18-19 flat highway unloaded.” But after two years, he got “13. said, “My  truck is currently getting an average of 11-12 MPG”(10).

Another LB7 53mpg hand calculated per tank on flat highway unloaded” (11).

Why Is My Duramax Getting Bad Fuel Mileage? – The Reasons & Solutions

So, you have the basic ideas about the MPG stuff of Duramax engines, right? Let me discuss why you may get bad fuel mileage in these engines (like the reports mentioned earlier).

First Reason – Fuel Injection System Efficiency

If the Duramax fuel injection system has issues with the fuel lift pump, you will get lousy mileage. After all, the pump is an essential part of the injection fuel system (12).

Over time, debris bypasses the filter and enters the fuel system. Then, it causes fuel injection issues and gradually reduces the fuel economy. You may read this report (13).

Excessive wear of diesel fuel injectors can lead to their failure. It can impact the fuel efficiency of Duramax engines (14). Again, a defective injector harness can cause a short or open in the wiring. As a result, you may get low fuel efficiency (15).


You can follow several strategies to improve fuel mileage in a Duramax fuel injection system. My first suggestion would be to upgrade the exhaust system. More precisely, you can install a 4″ exhaust with a turbo-back configuration. It will enhance power and mileage (16).

Another way can be using a tuner like Bully Dog GT. It offers multiple settings for different driving conditions. Also, it can increase power and give you the mileage you want (17).

Second Reason –Clogged Air Filtration System

If your vehicle has a dirty or clogged engine air filter, it will reduce the amount of airflow into the engine. As a result, it will cause the engine to work harder. It will require more fuel to maintain performance (18, 19, 20).


Regular air filter maintenance is essential for optimal fuel efficiency in Duramax engines. Still, if you can find a cloggedair filter is the reason, replace it with a new one. It can give you a boost of 20-50 HP engine performance. Also, fuel efficiency will improve with it (21).

Third Reason –Incorrect Tire Pressure

Tire pressure significantly affects fuel mileage in a Duramax truck. But if it is incorrect (under or over-inflated), it can cause poor fuel efficiency. It is estimated that for every 10 PSI below the recommended amount, fuel efficiency decreases by roughly 1% (22).

Suppose you have under-inflated tires. Then, it can lower fuel mileage by 0.2% for each 1 PSI drop in the standard pressure of any tire (23). Alternatively, over-inflated tires can make things more complicated. It depends on factors like speed (24).


You should always maintain the recommended tire pressure. It can improve fuel or gas mileage by up to 3% as the U.S. Department of Energy states (25). In this case, when pushing air inside the tires, follow the owner’s manual of the vehicle.

Fourth Reason – Fuel Quality & Additives

We often think we can save money by buying low-quality fuel for our vehicles. However, it can negatively impact the fuel mileage of a Duramax engine. After reading a discussion, I have found that brand-name fuels may provide better mileage than those from no-name stations (26).


You can find two grades of diesel for Duramax: grade #1 (1-D) and #2 (2-D) (27). Grade 1 performs relatively better than grade 2, particularly in winter. But if you want to carry heavy loads long distances, you can select grade 2 diesel.

Fifth Reason – Incorrect Camber and Steering Alignment

According to Dunn Tire, “camber is the inward or outward tilt of the front tires as viewed from the front of the vehicle” (28). It distributes vehicle loads across the entire tread. But if it is incorrect, it can cause tire wear on one edge. As a result, the truck will pull to the most positive camber’s side.

With this, the steering alignment also causesissues like noisy steering, uneven tire wear, and further tire issues. Overall, incorrect alignment of both components can consume more fuel to give you the correct mileage.


Dunn Tire suggestskeeping the camber alignment to zero. It provides the most uniform tire wear over time. But it may reduce performance when you turn the vehicle. However, I suggest you visit a mechanic to adjust the steering and camber. Otherwise, you may make things worse.

Sixth Reason – Bad Driving Habits

How do you drive your truck with a Duramax engine? Well, it may affect fuel mileage in this engine. If you are prone to accelerate swiftly, you must pour more fuel. Similarly, applying the brakes more frequently can also reduce fuel economy.


Increasing the highway cruising speed can enhance fuel economy by 20%. For example, if you increase it from 55mph to 75mph, you will save more fuel (29). So, stop thinking like you can save energy by driving slowly.

Seventh Reason – Dirty or Faulty Oxygen Sensor

A bad oxygen sensor can throw off the fuel delivery and combustion systems. It will disrupt the air/fuel ratio mixture. As a result, the fuel mileage will be hampered (30).


To maintain fuel efficiency, you should replace the oxygen sensor after driving every 60-70K miles (31).

Final Words

So, how was the discussion on Duramax getting bad fuel mileage?

I hope you enjoyed learning about why you get bad fuel economy in your truck.

Throughout the blog, I have explained different causes that can reduce engine performance. Also, I have provided solutions to each reason that I hope you will follow.

Always use the correct oil to maintain the health of the engine. Also, regular maintenance can be beneficial.Try to stay updated by taking part in the discussions of other Duramax owners online. It will help you in further troubleshooting.

Do not forget to leave your valuable feedback. I will help you with the necessary information.

Thank you.

Leave a Comment