12 Causes | Why is My Duramax Getting Bad Fuel Mileage? [+Solutions]

The mileage of the Duramax engine varies from model to model.

For example, the mileage of LB7 and the Mileage of L5P are not the same.

The average mileage of L5P and LML is slightly higher than the average of other Duramax engines.

L5P and LML’s average mileage is 24 mpg on highways and 18 mpg in cities.

But the average mileage of different models of Duramax is 20 mpg on the highways and 14 mpg in cities.

Your mileage should be within this range.

If your mileage is below 14 mpg in the city and below 20 on the highway, you are getting a bad fuel mileage.

Here are 12 reasons why your Duramax is getting bad fuel mileage.

01. Incorrect Tire Pressure and Worn Tire Tread

Incorrect Tire Pressure and Worn Tire Tread

Incorrect tire pressure is one of the main reasons for bad fuel mileage.

Most vehicle owners are indifferent to tire pressure.

They don’t know the company recommended tire pressure.

GM recommends 30 psi tire pressure in front and 35 psi in the rear tires.

But I suggest 35 psi in all four tires to help gain a good mileage.

Worn tire thread is also liable for reduced MPG.

Possible Solution

If you want extra mileage without affecting the braking system or damaging your truck’s tires, keep 5 psi extra tire pressure than GM recommends.

Read also: lb7 duramax problems.

02. Lower Quality Diesel Oil

Lower Quality Diesel Oil

There are 3 types of diesel.

  1. Petroleum diesel
  2. Synthetic diesel
  3. Biodiesel

Use biodiesel in the GM Duramax engine.

There are 2 grades of diesel, grade #1 (1-D) and #2 (2-D) diesel.

Grade #1 diesel is also known as winter diesel cause it performs better than grade #2 diesel in winter.

So, use grade #1 diesel in the winter.

And use grade #2 diesel for carrying heavy loads and long distances.

Grade #1 (or 1-D) is more fuel efficient than grade #2 diesel.

Possible Solution

Use 1-D or grade #1 diesel for better fuel mileage.

03. Incorrect Camber and Steering Alignment

Incorrect Camber and Steering Alignment

If the camber and steering alignment is incorrect, it will reduce the fuel economy.

4 Symptoms of Incorrect Camber/Steering Alignment

  1. Noisy steering
  2. Pulling to one side [right or left]
  3. Uneven tire wear
  4. Squealing tires

Possible Solution

Take the vehicle to the Dealer or to an experienced mechanic.

Don’t try to adjust the camber by yourself.You may do more damage without realizing it.

Make sure you align vehicle’s wheels periodically.

04. Air Conditioner

Air Conditioner

Air conditioner decreases fuel mileage as the conditioner consumes around 15% to 20% extra fuel.

It will create overpressure on the engine and decrease the MPG.

Possible Solution

Do 3 things.

  1. Turn off the AC when not needed
  2. Use the re-circulation button
  3. Park the vehicle in the shade

05. Driving Habits

Driving Habits

Driving habits are also responsible for fuel mileage increases or decreases.

Swift acceleration and using brakes more often consume more fuel.

Driving slowly or at a medium pace will increase by 15% to 20% fuel mileage.

Possible Solution

Drive slowly by maintaining the traffic rules.

If you drive within 10 MPH to 50 MPH, your fuel consumption will be low than the average.

06. Using the Wrong Engine Oil Grade

Using the Wrong Engine Oil Grade

Diesel oil has different grade levels.

There are 03 types of engine oil grade levels that GM recommends for Duramax engines.

  1. SAE 5W-40
  2. SAE 15W-40
  3. SAE 10W-30

You must know which engine oil GM-recommended for your Duramax.

Using the wrong grade of engine oil causes white smoke to blow from the exhaust.

Possible Solution

Check the owner’s manual to find out the recommended engine oil grade.

Always use recommended grade synthetic engine oil.

07. Clogged, Damaged, or Faulty Fuel Injector

Clogged, Damaged, or Faulty Fuel Injector

Duramax has an inbuilt bad fuel injector issue.

And the irony is GM doesn’t provide a warranty for the fuel injector.

The fuel injector determined the fuel flow volume into the combustion chamber.

A faulty fuel injector directly affects the engine and reduces its fuel economy.

7 Symptoms of a Fuel Injector Failure

  1. Check engine warning light
  2. Rough idle
  3. Misfiring or vibrating engine
  4. Fuel leak
  5. Poor fuel economy
  6. Engine stalling
  7. Engine overheating

If the symptoms appear in the fuel injector, it will be a reason for bad fuel mileage.

Read also: Duramax CP3 pump failure symptoms.

Possible Solution

Cleaning the fuel injector with a cleaner won’t solve the issue.

There is nothing you can do but change the fuel injectors.

There is a huge debate about whether to change only the bad fuel injector or the whole set of fuel injectors.

There are 8 fuel injectors in Duramax 6.6 engine.

From my past experience, I can confidently say if you change only the bad fuel injector, the fuel injector issue will arise again within 15k miles.

Cause old, and new fuel injectors don’t work properly.

So, as a mechanic, I suggest replacing the full-set fuel injector.

08. Dirty/Faulty MAF Sensor

Dirty MAF Sensor

A mass airflow sensor (MAF) calculates the air that enters the engine and provides actual data to the ECU.

7 Symptoms of a Dirty/Faulty MAF Sensor

  1. Check engine light
  2. Black exhaust
  3. Hard starts
  4. Hesitation
  5. Trouble accelerating
  6. Rough idle
  7. And finally, bad fuel mileage

Possible Solution

There are 2 solutions.

1. Cleaning

Clean the dirty MAF sensor with cleaner and reinstall it.

Clean the sensor every 4 months.

Follow this 3 steps method to clean the MAF sensor. 

  1. Remove the sensor
  2. Clean the sensor
  3. Dry and reinstall the sensor

2. Replace the MAF Sensor

As the sensor is very sensitive, GM doesn’t recommend cleaning the MAF sensor.

Instead, GM suggests changing the MAF sensor.

MAF sensor cost around $36.

09. Dirty/Faulty Oxygen Sensor

Faulty Oxygen Sensor

A dirty/faulty oxygen sensor affects the fuel mileage negatively.

6 Symptoms of a Dirty/Faulty Oxygen Sensor

  1. Misfiring or stalling
  2. Check engine light
  3. Bad engine performance
  4. Bad fuel mileage
  5. Engine overheating
  6. Continually failed emission tests

Possible Solution

GM doesn’t recommend cleaning the Oxygen sensor.

So, to eliminate such problems, replace the oxygen sensor every 60,000 to 70,000 miles.

10. Dirty Engine Air Filter

Dirty Engine Air Filter

A dirty air filter reduces the amount of airflow into the engine and increases the consumption of fuel.

06 Signs of a Dirty Air Filter

  1. Strange engine noise
  2. Reduced engine power
  3. Engine misfires
  4. Reduced fuel mileage
  5. The filter looks muddy
  6. Reduced HP

Possible Solution

Clean the air filter with an air blower every 7000 miles.

And change the engine air filter every 15000 miles.

11. Worn Brake Pads

Worn Brake Pads

Worn brake pads are another reason for bad fuel mileage.

6 Symptoms of Worn Brake Pads

  1. Clicking noise
  2. Longer brake distance
  3. Squealing noise
  4. Vibrating when braking
  5. Spongy brakes
  6. During braking truck’s nose pulls to one side

Possible Solution

As a mechanic, I recommend changing brake pads every 30000 miles.

It will help you to gain good fuel mileage and a safe braking system.

12. Engine Idling

Engine Idling

Idling a truck or car consumes up to a half gallon of fuel for bigger engines every hour.

In cold weather, the engine consumes more fuel to reach sufficient temperature.

In the winter, the air is denser.

When you drive in a colder area, the aerodynamic drag increases on your truck and decreases the fuel mileage.

Possible Solution

Turn off the ignition in a red light or traffic jam.

Don’t idle your truck unnecessarily.


How many miles per gallon does a 6.6 Duramax get?

A Duramax 6.6L engine generally gets 13 mpg in the city and 18 mpg on the highway.

If you want to get better fuel mileage with a 6.6 Duramax engine, follow these tips:

  • Always shift gears smoothly.
  • Use high-quality synthetic oil.
  • Avoid idling.
  • Ensure maintenance according to the manufacturer’s recommendations.
  • You can also install a tune, which improves fuel efficiency around 1-2 mpg.

What is the average MPG for a Duramax diesel?

Average mpg of a Duramax diesel engine mostly depends on the engine type and model.

Some Duramax engines are designed for heavy-duty tasks and they provide lower fuel efficiency.

While some Duramax engines are designed for better fuel efficiency.

For example, a heavy-duty 6.6L Duramax gets 17-18 mph on the highway and 13 mpg in the city.

On the other hand, a fuel efficient 3.0L Duramax diesel engine provides 23 mpg in the city and 33 mph on the highway with a 2WD drive configuration.

Fuel efficiency also depends on the driving configuration (2WD, 4WD, Cab size) and driving habits.

What MPG does a 3.0 Duramax get?

As previously stated, fuel economy is affected by driving configuration, so 3.0L Duramax MPG varies depending on the driving option.

A 3.0L Duramax engine with 2WD provides 23 mpg/city and 33 mpg/highway.

In contrast, 3.0L Duramax with 4WD provides 29 mpg on the highway and 23 mpg in the city.

Final Thoughts

As there are many reasons for bad fuel mileage in your Duramax.

First, identify the cause, then solve the issue.

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