How Much Diesel Exhaust Fluid (DEF) Does A Truck Use?

Diesel Exhaust Fluid (DEF) is a crucial component in modern diesel vehicles, introduced to comply with US Environmental Protection Agency (EPA) regulations that began in 2010. It's a solution made up of 32.5% urea and 67.5% deionized water, used to reduce harmful nitrogen oxide (NOx) emissions. The fluid is injected into the exhaust system through Selective Catalytic Reduction (SCR), where it helps break down NOx into harmless nitrogen and water.

DEF is typically stored in a dedicated tank, which is marked with a blue cap and can hold about 5-6 gallons on most trucks. Engine maker Cummins says that on average, a diesel truck uses it at a rate of about 3% to 5% of its total diesel fuel consumption. If you've got a solid engine getting around six miles per gallon, some back-of-the-envelope math says that translates to roughly five to eight gallons of DEF every 1,000 miles. For a typical 6.7L Powerstroke engine, the rough benchmark is about one gallon of DEF for every 200 gallons of diesel. This means under normal use, you're looking at a DEF refill every 5,000 to 7,000 miles, depending on driving conditions and fuel consumption.

If you run out of DEF, your truck might enter a "limp mode" where speeds get capped anywhere between 5-50 mph. That should just be enough to reach a DEF pump, but not much further. You can also top up at truck stops, auto parts stores, and even some big box retailers.

Keeping your DEF system in good shape doesn't require a PhD in chemistry, but a little know-how goes a long way. First, make it a habit to top off your DEF before long drives. Trucks built after 2010 are equipped with dashboard indicators that start warning you when DEF levels dip below 10%. Manual checks are also easy because you can find the DEF tank usually next to the diesel saddle tank or tucked near the spare tire in smaller vehicles.

Storage is another key factor you'll need to pay attention to. DEF has a shelf life of about two years, but that's assuming it's stored between 12°F and 86°F and out of direct sunlight. Heat causes evaporation, while cold causes it to freeze — it starts solidifying at 12°F. The good news is, DEF freezes evenly (water and urea in sync), and once thawed, it's good to go. Just be mindful that frozen DEF expands by about 7%, so give the fluid time to drain back into the tank before shutting off the battery in winter conditions. You also shouldn't add additives or anti-gel agents to the mix since it can mess up your SCR system.