How Long Does A Tank of DEF Last? Defining the Lifespan of Diesel Exhaust Fluid

When it comes to maintaining your diesel engine, one of the most important considerations is the DEF tank. DEF, or Diesel Exhaust Fluid, is an essential fluid component that works with your vehicle’s exhaust system to reduce emissions. But with so many variables at play, it can be hard to tell how long a single tank of DEF will last you. That’s why we’ve put together this informative article to help you get a better sense of what to expect.

First and foremost, it’s important to understand that the amount of DEF your vehicle will consume will depend on a lot of factors. These can include everything from the size of your DEF tank to the frequency and duration of your driving, outside temperature, and even the type of driving you’re doing. Whether you’re towing a heavy load or just cruising around town, all of these factors will impact the rate at which your DEF is consumed.

Of course, all of this variability can make it tough to provide a concrete answer. However, based on our research and experience, we can confidently say that a tank of DEF should last you upwards of several thousand miles. Of course, exactly how many miles can vary quite a bit – but as long as you’re mindful of your driving habits and keep an eye on your DEF guage, you shouldn’t have too much trouble staying on top of it.

What is DEF (Diesel Exhaust Fluid)?

DEF stands for Diesel Exhaust Fluid. It is a non-hazardous solution, made up of 32.5% high-purity urea and 67.5% de-ionized water, that is sprayed into the exhaust stream of diesel vehicles in order to reduce the amount of harmful emissions released into the environment. DEF is a key component in a vehicle’s Selective Catalytic Reduction (SCR) system, which converts harmful nitrogen oxide (NOx) emissions into harmless nitrogen and water vapor.

Benefits of DEF

  • Reduces harmful NOx emissions
  • Improves air quality
  • Helps diesel vehicles comply with environmental regulations
  • Reduces engine wear and tear, leading to longer engine life

How DEF Works

When diesel fuel is burned, it produces harmful emissions like NOx. The SCR system, which includes the DEF injection system, reduces these emissions by converting NOx into harmless nitrogen and water vapor. DEF is injected into the exhaust stream, where it mixes with the hot exhaust gases and reacts with the NOx catalyst in the SCR system. This chemical reaction reduces NOx emissions by up to 90%. As a result, DEF helps diesel vehicles meet strict emissions standards set by the Environmental Protection Agency (EPA) and California Air Resources Board (CARB).

How Long Does a Tank of DEF Last?

The amount of DEF needed by a vehicle depends on a number of factors, including the size of the vehicle’s DEF tank, the amount of driving the vehicle does, and the vehicle’s fuel efficiency. On average, a full tank of DEF will last between 2-4 weeks. Some newer diesel vehicles have a DEF gauge that indicates how much DEF is left in the tank, while others use a warning light or message when the DEF tank is low.

DEF Tank Size DEF Consumption Rate Tank Lasts
2.5 gallons 2-3% of diesel fuel consumption 800-1200 miles
5.0 gallons 2-3% of diesel fuel consumption 1600-2400 miles
10.0 gallons 2-3% of diesel fuel consumption 3200-4800 miles

It is important to note that driving without DEF or with a low DEF level will result in reduced engine performance and could eventually lead to a complete shutdown of the vehicle. It is recommended to keep an eye on the DEF tank level and refill it as necessary to ensure optimal vehicle performance and to avoid costly repairs.

Composition of DEF

Diesel Exhaust Fluid (DEF) is a non-toxic, colorless, and odorless mixture of 32.5% urea and 67.5% deionized water. It meets the ISO 22241 standard and is a crucial ingredient in many diesel engines vehicles.

Benefits of DEF

  • DEF helps to reduce diesel emissions and promote better air quality by converting nitrogen oxides (NOx) into harmless nitrogen and water vapor.
  • It increases fuel efficiency and lengthens the lifespan of engines by decreasing the amount of particulate matter released from the tailpipe.
  • DEF reduces the cost of engine maintenance and repairs by preventing corrosion and other harmful deposits.

Usage and Shelf Life

DEF is typically consumed at a rate of 2-3% of diesel fuel consumption, meaning that a tank of DEF can last anywhere from 1,500 to 5,000 miles depending on the vehicle’s fuel economy and driving conditions. When stored properly, DEF has a shelf life of about two years. However, it is important to keep it in a temperature-controlled environment (preferably between 12°F and 86°F) and out of direct sunlight to prevent it from degrading.

Composition of DEF by Weight

Component Weight
Urea 32.5%
Deionized Water 67.5%

Overall, the composition of DEF is straightforward and effective in reducing emissions from diesel engines. Its distribution and usage have helped to promote environmentally friendly driving practices and ensure the longevity of engines.

How DEF works to reduce emissions

Diesel Exhaust Fluid (DEF) is a non-toxic, colorless, odorless, and non-flammable solution that is used in diesel engines to reduce the amount of harmful emissions released into the air. It is a necessary component of selective catalytic reduction (SCR) systems that are now required to meet the strict emissions standards set by the US Environmental Protection Agency (EPA) in diesel engines over a certain size.

DEF is composed of two main elements: urea and deionized water. When this solution is injected into the exhaust stream of a diesel engine, it reacts with the exhaust gases and converts nitrogen oxide (NOx) into harmless nitrogen and water vapor. This process is known as SCR, and it is highly effective in reducing emissions as much as 90 percent.

Benefits of using DEF

  • Compliance with strict emissions standards
  • Reduced environmental impact
  • Improved fuel efficiency and engine performance

The importance of using high-quality DEF

To ensure that DEF functions properly and delivers optimal performance, it is crucial to use high-quality DEF that meets the ISO 22241 standards. Using low-quality or contaminated DEF can cause damage to the SCR system and lead to costly repairs or even engine failure. Additionally, using non-compliant DEF can result in penalties and fines from the EPA.

It is recommended to only purchase DEF from reputable suppliers and to check for the ISO 22241 certification on the label or package. DEF should also be stored properly in a cool, dry, and clean area to prevent contamination and degradation.

DEF usage rates

The usage rate of DEF can vary depending on the size of the diesel engine, the operating conditions, and the amount of miles driven. As a general rule of thumb, DEF consumption is usually around 2 to 3 percent of diesel fuel consumption. This means that for every 100 gallons of diesel fuel used, approximately 2 to 3 gallons of DEF will be required.

Diesel Fuel DEF Consumption
100 gallons 2-3 gallons
500 gallons 10-15 gallons
1000 gallons 20-30 gallons

It is important to check the DEF levels regularly and to replenish as needed to prevent running out and potentially damaging the SCR system.

Vehicles that require DEF

Many modern diesel vehicles require Diesel Exhaust Fluid (DEF) in order to meet emissions standards. DEF is used to reduce the emissions of harmful nitrogen oxides (NOx) that are released by diesel engines. DEF is not required for older diesel vehicles, but for newer ones it is essential to maintain proper engine function and meet emissions regulations.

  • Heavy-duty trucks: Trucks that weigh over 8,500 pounds and are model year 2010 or newer require DEF.
  • Passenger cars: Certain diesel sedans and SUVs, such as the Mercedes-Benz Sprinter and the Volkswagen Jetta TDI, require DEF.
  • Buses: Large buses, including school buses and intercity coaches, require DEF.

It is important to note that not all diesel vehicles require DEF. Some older diesel engines do not have the necessary systems to require it, and not all new models require it either. If you’re not sure whether your diesel vehicle requires DEF, consult the owner’s manual or contact the manufacturer.

For vehicles that do require DEF, the amount used will depend on a variety of factors, including the size of the vehicle, the engine size, and how the vehicle is driven. Generally, a tank of DEF can last anywhere from 2,000 to 10,000 miles depending on these factors.

Vehicle Type DEF Usage
Heavy-duty truck 2-4% of diesel fuel consumption
Passenger car Approximately 2.5 gallons for every 800 miles driven
Bus Approximately 2.5 gallons for every 200 miles driven

If your vehicle runs out of DEF, it will likely go into a “limp mode” where the engine power is reduced to protect the emissions system. It is important to keep an eye on your DEF levels and refill the tank when necessary to avoid this situation.

Average size of DEF tank in vehicles

DEF tank size varies depending on the vehicle size and model. However, on average, most diesel vehicles come equipped with a DEF tank that ranges from 2.5 to 8 gallons.

  • Small cars and SUVs typically have DEF tanks that range from 2.5 to 4 gallons in size.
  • Medium-size trucks and SUVs usually have DEF tanks with a capacity of 5 to 6 gallons.
  • Large commercial vehicles, such as semi-trucks and buses, often have DEF tanks that range from 6 to 8 gallons.

It’s important to note that larger DEF tanks don’t necessarily mean longer DEF life. The longevity of your DEF depends on various factors such as driving habits, terrain, and weather conditions.

Generally speaking, the average tank of DEF will last you about 2,400 miles.

DEF Tank Size DEF Usage rate DEF Life
2.5 gallons 1 gallon per 800 miles 2,400 miles
4 gallons 1 gallon per 800 miles 3,200 miles
5 gallons 1 gallon per 800 miles 4,000 miles
6 gallons 1 gallon per 800 miles 4,800 miles
8 gallons 1 gallon per 800 miles 6,400 miles

Remember, it’s crucial to check your DEF level regularly to avoid any problems with your vehicle’s emissions system. Make sure to top off your DEF tank before it’s completely empty to prevent any damage to your engine and comply with EPA regulations.

Fuel efficiency impact of using DEF

The use of diesel exhaust fluid (DEF) in select diesel engines is known to have both environmental and economic benefits. The primary function of DEF is to reduce the amount of harmful emissions produced by diesel engines. This is done by injecting DEF into the engine’s exhaust system as a part of the selective catalytic reduction (SCR) process. The chemical reaction that occurs causes nitrogen oxide (NOx) emissions to be converted into harmless nitrogen and water vapor.

While the environmental benefits of using DEF are significant, there are also economic benefits. DEF usage results in increased fuel efficiency, which can save money for both individual drivers and commercial fleets. The following factors impact fuel efficiency when using DEF:

  • The type of vehicle or equipment being used: DEF usage can increase fuel efficiency in vehicles or equipment with diesel engines that require the use of DEF for SCR systems.
  • The quality and quantity of DEF used: Using a high-quality DEF can maximize the fuel efficiency benefits. It’s also important to ensure that the proper amount of DEF is used, as overuse or underuse can impact fuel efficiency.
  • Driving conditions: Fuel efficiency will vary depending on the driving conditions, such as the type of roads, heavy traffic, and terrain. However, in most cases, using DEF will improve fuel efficiency.

Studies have shown that fuel efficiency increases anywhere from 3% to 5% when using DEF. This equates to savings in fuel costs for commercial fleets and individual drivers alike. The following table outlines the estimated savings in fuel costs with the use of DEF:

Vehicle Type Average Annual Fuel Cost DEF Usage Cost (5% of Fuel Cost) Total Annual Fuel Cost with DEF Annual Savings with DEF
Class 6 Truck $40,500 $2,025 $38,475 $2,025
Class 8 Truck $70,000 $3,500 $66,500 $3,500
Off-Road Equipment $22,000 $1,100 $20,900 $1,100

Overall, using DEF can have a positive impact on fuel efficiency, resulting in cost savings and environmental benefits.

Factors that affect DEF consumption rate

SCR systems require the use of DEF to reduce emissions of nitrogen oxides (NOx) from diesel engines. The consumption of DEF varies depending on several factors, including:

  • Engine size and power output: Larger engines consume more DEF than smaller engines due to their higher power output.
  • Operating conditions: DEF consumption increases during high load or high-speed operation because the engine emits more NOx.
  • Temperature: DEF consumption increases in colder temperatures because it requires more heat to vaporize and reach the SCR system.
  • Quality of DEF: Low-quality DEF may contain impurities that can damage the SCR system and require more frequent refilling.
  • Vehicle type: Heavy-duty trucks, buses, and other commercial vehicles consume more DEF than passenger cars due to their larger engine size and longer operating hours.
  • Route and driving behavior: DEF consumption can vary depending on the route and driving behavior, such as stop-and-go traffic or long-haul driving.
  • Load capacity: Vehicles carrying heavy loads consume more DEF due to their higher power output and increased NOx emissions.

DEF consumption rates

DEF consumption rates vary depending on the factors mentioned above, but the typical consumption rate is between 2-4% of diesel fuel consumption. This means that for every 100 gallons of diesel fuel consumed, 2-4 gallons of DEF will be used.

Engine Size DEF Consumption Rate
3 – 6 liters 2 – 3% of diesel fuel consumption
6 – 12 liters 3 – 3.5% of diesel fuel consumption
12 – 16 liters 3.5 – 4% of diesel fuel consumption

It’s important to note that running out of DEF can cause the SCR system to shut down, leading to engine derating or even failure to start. Therefore, it’s essential to keep track of DEF levels and refill as necessary to avoid potential downtime or costly repairs.

Average Mileage per Gallon of DEF

Understanding the average mileage per gallon of DEF can help drivers estimate how long their tank of DEF will last. Diesel exhaust fluid is typically consumed at a rate of 2-4% of diesel fuel consumed, meaning that for every 100 gallons of diesel fuel used, 2-4 gallons of DEF will be needed.

  • The average mileage per gallon of DEF can vary depending on the vehicle and driving conditions. On average, a gallon of DEF will last around 3000 miles of driving.
  • In addition to driving conditions, the age and condition of the vehicle, as well as the quality of the DEF used, can affect the average mileage per gallon.
  • It is important to note that running out of DEF can cause the vehicle to go into a limited performance mode, which may result in decreased mileage and increased maintenance costs. Keeping an eye on DEF levels and refilling the tank as needed can help prevent these issues.

Here is a table showing approximately how many gallons of DEF will be used for varying amounts of diesel fuel consumed at a rate of 2-4%:

Diesel Fuel Consumed DEF Used (at 2%) DEF Used (at 4%)
100 gallons 2 gallons 4 gallons
500 gallons 10 gallons 20 gallons
1000 gallons 20 gallons 40 gallons

By estimating the average mileage per gallon of DEF and keeping an eye on DEF levels, drivers can ensure their vehicles operate at their best and avoid any potential issues from running out of DEF.

How to Store DEF Properly

DEF (Diesel Exhaust Fluid) is a vital component in modern diesel-powered engines, and its quality and purity can greatly affect the performance of your vehicle. One important factor that can impact the quality of DEF is how it is stored.

1. Choose the Right Container

When storing DEF, it’s important to choose the right container to avoid contamination. DEF should always be stored in a container made of high-density polyethylene (HDPE) or a non-reactive material. Never use a container that has held other fluids, such as oil or antifreeze, as this can contaminate the DEF and cause problems with your vehicle.

2. Store in a Cool and Dry Place

DEF should be kept in a cool and dry place, away from direct sunlight. When exposed to high temperatures or sunlight, DEF can start to break down and lose its quality. Ideally, DEF should be stored at a temperature between 15°F and 77°F, and should be kept out of freezing temperatures. Moisture can also contaminate the DEF, so make sure the storage area is dry.

3. Avoid Contaminants

Contaminants such as dust, dirt, and water can reduce the quality of DEF and cause damage to your vehicle’s engine. When filling or storing DEF, make sure the container and the surrounding area are clean and free of any contaminants. Never mix DEF with other fluids or additives, as this can also cause contamination.

4. Monitor Expiration Dates

DEF has a finite lifespan, and its quality can degrade over time. Make sure to monitor the expiration date of the DEF and use it before it expires. It’s important to note that DEF can only be stored for up to one year if the temperature is below 86°F, or six months if the temperature is above 86°F.

5. Use a Closed System

Avoid open systems when storing or transporting DEF, as this can allow contamination. Use a closed system to help keep the DEF pure and protect it from unwanted contaminants. If you must use an open system, be extra cautious and clean the system thoroughly to avoid any potential contamination.


  • Choose a container made of HDPE or non-reactive material
  • Store in a cool and dry place between 15°F and 77°F
  • Avoid contaminants such as dust, dirt, and water
  • Monitor expiration dates and use before it expires
  • Use a closed system to avoid contamination

DEF Storage Temperature Limits

DEF Temperature Shelf life
15°F to 77°F Up to one year
86°F and above Up to six months

Cost of DEF per gallon and total cost of operation over time.

When it comes to operating a vehicle that utilizes DEF, it is important to consider the cost of the fluid per gallon as well as the total cost of operation over time. DEF prices can vary depending on location and supplier but the average cost per gallon is around $2.70. This may seem like a considerable expense but it is important to keep in mind that the fluid is not used up as quickly as gasoline or diesel fuel.

One way to determine the total cost of operation for DEF is to calculate the amount of fluid used over a certain period of time. Most vehicles using DEF have an on-board computer system that monitors the fluid level and usage. By accessing this system and recording the amount of DEF used over a specific distance or period, one can estimate the total amount needed for a year of operation. For example, if a truck travels 100,000 miles per year and uses an average of 3 gallons of DEF per 1,000 miles, it would require 300 gallons of DEF per year.

Factors that impact DEF cost

  • Geographic location: DEF prices can vary depending on the area and region in which it is purchased.
  • Supplier: Suppliers can have different pricing structures and incentives for bulk purchases or repeat customers.
  • Vehicle usage: The amount of DEF used can vary based on driving conditions and vehicle load.

Calculating total cost of operation

When calculating the total cost of operation for a vehicle using DEF, it is important to consider all associated costs including the cost of the fluid, maintenance costs, and potential fines for non-compliance with emissions regulations. The chart below provides an example of how to calculate the total cost of operation for a typical semi-truck

Expense Cost Frequency Total Cost Per Year
DEF fluid $810 annually $810
Maintenance/Repair $5,000 annually $5,000
Oil Changes $800 twice per year $1,600
Driver Wages $55,000 annually $55,000
Fines for non-compliance $10,000 annually $10,000
Total $71,410

As seen in the chart above, while DEF fluid cost is a factor, it is only a small portion of the total expense associated with operating a vehicle. By properly maintaining the vehicle and following emissions regulations, these additional costs can be minimized.

How Long Does a Tank of DEF Last? FAQs

1. How long does a tank of DEF last?

The life of a DEF tank depends on the size of the tank and how often you use DEF. In general, a full tank of DEF can last up to a few thousand miles.

2. How do I keep track of my DEF levels?

Most modern vehicles have DEF gauges to measure DEF levels. However, you can also check the DEF levels manually by looking at the DEF indicator on the dashboard.

3. What happens if my DEF tank runs out?

If your DEF tank runs out, your vehicle’s engine power will be reduced, and your vehicle may shut down completely.

4. How often should I refill a DEF tank?

You should refill your DEF tank when the DEF levels get low. It is recommended to check and refill your DEF tank every time you change your oil.

5. Can I mix DEF with other fluids?

No, it is not recommended to mix DEF with any other fluids. Doing so can damage your vehicle’s engine and exhaust system.

6. How do I dispose of leftover DEF?

If you have leftover DEF, you can take it to a recycling center or dispose of it in accordance with your local regulations.

7. What happens if I use contaminated or expired DEF?

Using contaminated or expired DEF can damage your vehicle’s engine and exhaust system. It is best to avoid using expired or contaminated DEF at all costs.

Closing Thoughts: Thanks for Reading!

We hope this article has provided you with helpful information on how long a tank of DEF lasts. Remember to keep an eye on your DEF levels and refill as needed to keep your engine running smoothly. Thanks for reading, and we hope to see you again soon for more helpful tips and information.