DEF cost and usage per gallon of diesel

Diesel exhaust fluid is a small line on a trucking budget, but it is a line every SCR-equipped truck pays on every mile. It also causes some of the most frustrating breakdowns when it is neglected: a derated engine on the side of the road because the tank ran dry or the fluid went bad. This guide covers how much DEF a truck uses, what it costs, how to store it, and how to fold it into your cost per mile.

How the SCR system uses DEF

Tap each part to see where DEF goes and what can go wrong.

DEF is a solution of 32.5 percent high-purity urea in deionized water, made to the ISO 22241 standard. In the hot exhaust, the urea breaks down into ammonia, which reacts with nitrogen oxides on the SCR catalyst. The engine's controls measure the result and adjust how much DEF is sprayed.

How much DEF per gallon of diesel

Many modern engines use DEF at about 2 to 3 percent of their diesel consumption. The rate is not fixed: it rises with engine load and depends on the engine's calibration. Heavy loads, steep grades and high exhaust temperatures can push it up, while light highway cruising can bring it down.

A simple check: for a month, record every DEF fill and every diesel fill. Divide the DEF gallons by the diesel gallons. That is your truck's real dosing rate.

DEF cost per mile

DEF cost per mile = (miles / MPG) x dosing rate x DEF price / miles

For a truck getting 6.5 MPG, using DEF at 2.5 percent and paying $3.50 a gallon for DEF:

• Diesel per mile: 1 / 6.5 = about 0.154 gallons
• DEF per mile: 0.154 x 2.5% = about 0.0038 gallons
• DEF cost per mile: 0.0038 x $3.50 = about 1.3 cents

The price used is an EXAMPLE; use the price at your pumps. Over 100,000 miles a year, that is around $1,350. The DEF usage and cost calculator works out your numbers.

Where to buy DEF

• Pump DEF at truck stops is usually the cheapest way to buy it on the road and keeps the fluid out of open containers.
• Jugs and boxes cost more per gallon but are useful for top-ups and remote areas.
• Bulk totes and drums suit fleets with a yard, if stored properly.

Use DEF that is certified to ISO 22241, often shown with an API certification mark. Avoid funnels and containers that have held anything else; even small amounts of contamination can damage the system.

Shelf life, storage and freezing

DEF breaks down over time, faster in heat. Industry guidance gives longer shelf life at cooler storage temperatures and shorter life in hot storage. Practical rules:

• Store DEF sealed, out of direct sunlight, in a cool place.
• Buy packaged DEF with a recent date, and use older stock first.
• Do not store jugs in a hot cab or on the catwalk in summer for weeks.

DEF freezes at about 12 °F (minus 11 °C). Trucks have tank and line heaters, and frozen DEF thaws once the engine runs; freezing does not damage the fluid itself. Leave room in the tank and in jugs, because DEF expands when it freezes.

What happens if DEF runs out or goes bad

Engines with SCR are designed to warn the driver when DEF runs low or its quality is wrong, and then limit engine power if the problem is not fixed. The exact warnings and derate steps depend on the engine maker. A truck that derates on a load is a lost load, so treat the DEF gauge like the fuel gauge: top up at every fuel stop or every second one.

If the system reports poor DEF quality, do not just add more. Contaminated fluid may need to be drained and the system flushed by a technician.

DEF and other fluids

DEF is one of several fluids that keep a modern truck running. Engine oil is the biggest by volume at service time; our semi truck oil capacity chart shows typical capacities. Wheel ends need their own lubricant; see hub oil for a semi truck. For the parts side, our guide to semi truck parts explains where to buy filters and other service items, and the guide to semi truck maintenance cost puts all of it into a yearly budget.

Putting DEF into your rate

DEF is a small cost, but it is part of your cost per mile, along with fuel, tires and maintenance. A load that clears your floor rate with every cost included pays for the DEF it uses. When you know your full cost per mile, you also know what a dispatcher's fee does to it; our page on dispatch rates shows the fee in plain numbers.

DEF across a year: an EXAMPLE budget

Here is an EXAMPLE year for a single truck, using round numbers you should replace with your own:

• Miles: 110,000
• Fuel economy: 6.5 MPG, so about 16,900 gallons of diesel
• DEF dosing: 2.5 percent, so about 420 gallons of DEF
• DEF price: $3.50 a gallon at the pump

That comes to roughly $1,480 of DEF a year. Buying it in jugs at a higher price could push that well above $2,000. Neither number is large next to fuel, but both are real, and both are easy to forget when judging a load.

DEF quality problems and how to avoid them

Most DEF trouble comes from contamination or age. Common causes include:

• Wrong containers. Funnels, jugs or cans that once held oil, coolant or fuel leave residues that can poison the SCR catalyst.
• Tap water. Even small amounts of minerals in ordinary water can damage the system. DEF is made with deionized water for a reason.
• Heat. Jugs left in a hot cab or on a catwalk for weeks degrade.
• Old stock. Packaged DEF that has sat for a long time may be past its useful life.
• Wrong tank. Putting DEF in the fuel tank, or diesel in the DEF tank, is a serious and expensive mistake. Many DEF filler caps are blue and use a smaller nozzle size to help prevent it.

If you suspect bad DEF, stop adding more and have the system checked. A technician can test the fluid's urea concentration with a refractometer.

DEF crystals and dosing problems

White crystal deposits around the doser, the exhaust pipe or the DEF lines are a sign that DEF is not being dosed or vaporized properly. Causes can include a failing doser, low exhaust temperatures from long idling or short trips, or a system fault. Crystals can block the doser and lead to warning lights and derates. Do not just scrape them off and carry on; have the cause diagnosed.

Cold weather tips

In winter, DEF tanks and lines rely on heaters to thaw frozen fluid after start-up. If the heater fails, the system cannot dose and the truck will warn the driver. Keep jugs of DEF in the cab during very cold weather only if they will be used soon, since heat later shortens their life. Do not add anti-gel additives or any other chemical to DEF to stop it freezing; that ruins the fluid.

DEF and fuel economy

Engines with SCR can often be calibrated for better fuel economy, because the SCR system cleans up nitrogen oxides after combustion. In practice, the small cost of DEF is offset in part by fuel savings compared with older emissions approaches. That does not make DEF free, but it is one reason it should be viewed as part of the fuel bill rather than a separate penalty.

Checking your truck's DEF use

If your DEF use suddenly rises or falls compared with your records, something may have changed: a new load pattern, a different route, or a fault in the dosing system. A sharp drop can mean the system is not dosing properly, and a sharp rise can point to a sensor or calibration problem. Keep a simple log of DEF fills next to fuel fills, and mention any unusual change at your next service. It is a quick way to catch a problem before it becomes a derate on the road.

The emissions system and the law

SCR, the DPF and the rest of the aftertreatment system are required emissions equipment. Removing or disabling them, or running without DEF by defeating the system, is illegal under federal law and can bring heavy penalties. If the system has problems, the answer is diagnosis and repair by a qualified technician.

Where we fit

We dispatch trucks from 26 ft box trucks on up. We price every load against your full cost per mile, DEF included. Learn more about our dispatch service.