Diesel DEF Consumption: What to Expect on Modern Trucks

Diesel exhaust fluid (DEF) is a normal part of owning a modern diesel truck, but many owners are unsure how much they should be using. This guide sets realistic expectations for DEF consumption, tank size and cost. Actual figures vary with how you drive and tow, so treat these as typical ranges and refer to your owner's manual for your truck.

Which trucks use DEF

DEF applies to modern trucks with selective catalytic reduction (SCR), generally 2010 and newer depending on the manufacturer.

• Cummins: SCR arrived on the 6.7L around 2013 for pickups.
• Power Stroke: the 6.7L Power Stroke has used SCR since its 2011 launch.
• Duramax: SCR arrived on the LML around 2011.
• Older trucks: pre-emissions and pre-SCR diesels do not use DEF at all.

How much DEF you will use

• Rule of thumb: DEF is consumed at roughly 2 to 3 percent of the diesel you burn.
• In practice: that is often around 1 gallon of DEF per 30 to 50 gallons of diesel, or very roughly 500 to 800 miles per gallon of DEF depending on load.
• Towing raises it: heavy towing burns more diesel and therefore more DEF.

Tank size and refilling

• Tank size: most pickups carry roughly a 5-gallon DEF tank; larger on some configurations.
• Refill timing: the gauge and dash warnings tell you when to top off; do not ignore low-DEF warnings, which can lead to speed derates.
• Storage: DEF has a shelf life and does not like heat or freezing; buy what you will use and store it cool.

Cost and quality

• Cost: DEF is inexpensive relative to fuel; jugs and pump DEF are both widely available.
• Quality: use only certified DEF that meets ISO 22241; never substitute other fluids.
• Handling: keep it clean, since contamination can affect the SCR system and sensors.

If a low-DEF or SCR warning will not clear, or you need a NOx or DEF-related sensor for your truck, email sales@coredieselparts.com with your VIN.

Frequently asked questions

How often will I add DEF?

For light driving, many owners top off every few thousand miles. Heavy towing shortens that. Watching the DEF gauge is the easiest way to plan refills before a warning appears.

What happens if I run out of DEF?

The truck will warn you well ahead of time, then progressively limit speed until DEF is added. It is a protective derate, not damage, but it is inconvenient, so refill before the tank runs dry.

Does my older diesel need DEF?

No. DEF only applies to SCR-equipped trucks, generally 2010 and newer. Pre-SCR diesels have no DEF tank and use no DEF at all.