Diesel and DEF Basics: Efficiency, ULSD and Exhaust Fluid

Diesel vehicles use compression ignition instead of spark plugs, run on a denser fuel, and often travel farther per gallon than comparable gasoline vehicles. Most modern diesels also carry an emissions system that uses diesel exhaust fluid (DEF), which is a separate fluid to top up. Diesel is also priced separately from gasoline, so MPG alone does not settle which costs less to run.

Key takeaways

• fueleconomy.gov says diesel vehicles can often go about 20% to 35% farther on a gallon of fuel than their gasoline counterparts.
• Diesel fuel contains roughly 10% to 15% more energy than gasoline, per gallon, according to fueleconomy.gov.
• Highway diesel in the U.S. is ultra-low sulfur diesel (ULSD), which has a sulfur content of 15 parts per million or less.
• DEF is used by selective catalytic reduction systems to reduce nitrogen oxides, and it has to be refilled.
• Diesel and gasoline have separate prices, so cost depends on both MPG and the diesel price on the day.

What diesel fuel is

The EIA defines diesel fuel as the common term for distillate fuel oil sold for motor vehicles with a compression-ignition engine. Diesel engines power freight trucks, trains, buses, boats, farm equipment and construction vehicles as well as some cars and light trucks. U.S. refineries produce an average of 11 to 12 gallons of diesel fuel from each 42-gallon barrel of crude oil, according to the EIA. Diesel is produced from biomass materials too.

Why diesels can be more efficient

Two things combine. First, a gallon of diesel holds more energy than a gallon of gasoline, which fueleconomy.gov puts at roughly 10% to 15% more. Second, diesel engines are more efficient: fueleconomy.gov credits improved fuel injection and electronic engine control, and notes that diesel engines have more low-end torque than similar-sized gasoline engines. The result, in the agency’s words, is that diesel vehicles can often go about 20% to 35% farther on a gallon. Those are ranges for typical comparisons, not a rule for any one model. Compare real ratings on the vehicle pages in our car database.

Ultra-low sulfur diesel

Before 2006, high sulfur levels in diesel created harmful air pollution. The EPA implemented requirements to reduce sulfur content, and the diesel now sold for highway use in the U.S. is ULSD with a sulfur content of 15 parts per million or less. ULSD is what lets modern exhaust treatment systems work. fueleconomy.gov lists ultra-low sulfur diesel, biodiesel and advances in emission control technologies as the means by which diesels meet the same emissions standards as gasoline vehicles.

What DEF is for

Since 2010, the EPA says nearly all on-road diesel trucks and many types of nonroad equipment have used DEF as part of selective catalytic reduction (SCR) systems to reduce nitrogen oxides, a major contributor to air quality problems. DEF is a fluid you add periodically in a separate tank, not a fuel and not an additive for the diesel tank. Refills are therefore part of ownership for vehicles with SCR.

The EPA also notes that systems monitor DEF and can force a vehicle to drastically reduce speed or become inoperable if it runs out, which is why vehicles warn the driver in advance. The owner’s manual has the refill interval and tank location for each vehicle.

Diesel prices and cost

Diesel is sold at a different price than gasoline, and the gap moves over time. The EIA publishes weekly retail diesel prices, with taxes included, for regions and states. Because of that, a diesel that gets more miles per gallon does not automatically cost less to fuel. Gallonly uses diesel prices for diesel vehicles and gasoline prices for gasoline vehicles; see how annual fuel cost is calculated, and run your own numbers in the fuel cost calculator.

Diesel and EPA ratings

Diesel passenger vehicles are rated in MPG like gasoline ones, using the same tests described in how the EPA tests fuel economy. Heavy-duty diesel pickups above 8,500 pounds GVWR are a different case, covered in why heavy-duty pickups are not EPA-rated.

Not to be confused with

• Premium gasoline. A different fuel with a different property; see regular vs premium gas.
• E85. A gasoline-ethanol blend for flex-fuel vehicles; see E85 and flex-fuel vehicles.
• Biodiesel. Mentioned by fueleconomy.gov as one of the ways diesel emissions are reduced; follow the fueleconomy.gov diesel page for current detail on blends.