A semi-truck burns roughly 0.6 to 1.5 gallons of diesel per hour while idling, depending on engine size, ambient temperature, and whether accessories like air conditioning are running. Smaller trucks, like medium-duty delivery vehicles and pickups, use less, typically between 0.4 and 0.85 gallons per hour. Over a full workday or an overnight rest period, those numbers add up fast.
Semi-Trucks: 0.6 to 1.5 Gallons per Hour
A Class 8 semi-truck, the kind hauling freight on highways, consumes between 0.6 and 1.5 gallons of diesel every hour it sits idling. The wide range reflects real-world variation. A newer truck with a smaller engine idling on a mild day with no accessories running will be closer to 0.6. An older truck running the air conditioning in July heat will push toward 1.5 or higher.
At even the low end of that range, the costs are significant. Long-haul drivers frequently idle overnight to keep the cab warm or cool. Eight hours of idling at 0.8 gallons per hour burns 6.4 gallons of diesel. At roughly $3.50 to $4.00 per gallon, that’s over $20 per night, per truck. Across a fleet of 200 trucks, overnight idling alone can cost more than $1 million a year in fuel.
Medium and Light-Duty Trucks
Department of Energy data breaks down idle fuel consumption for smaller trucks. A gasoline-powered medium heavy truck (5 to 7 liter engine, around 20,000 to 26,000 pounds) burns about 0.84 gallons per hour. A diesel delivery truck of similar weight uses roughly the same, around 0.84 gallons per hour. Diesel tow trucks come in lower at about 0.59 gallons per hour, and medium heavy diesel trucks with larger 6 to 10 liter engines use approximately 0.44 gallons per hour with no accessory load.
That last number might seem counterintuitive. Bigger diesel engines burning less at idle than smaller gas ones? Diesel engines are inherently more efficient at low loads because they don’t use a throttle plate to restrict airflow the way gasoline engines do. They inject only as much fuel as needed, which at idle is very little relative to their displacement.
How Engine Size Affects the Rate
The EPA found a straightforward relationship between engine displacement and idle fuel consumption: for every additional 100 cubic inches of engine size, idle fuel use goes up by about 0.16 gallons per hour. Put another way, each extra 50 cubic inches adds roughly 0.08 gallons per hour at idle. So a V8 pickup with a 6.2-liter engine (about 378 cubic inches) will burn noticeably more fuel at idle than a four-cylinder work truck with a 2.7-liter engine (about 165 cubic inches), simply because the bigger engine has more cylinders to keep spinning and more friction to overcome.
Air Conditioning Nearly Doubles Idle Fuel Use
Running the AC while idling has a much larger effect than most people expect. Research published in the Proceedings of the Institution of Mechanical Engineers found that air conditioning can increase fuel consumption by up to 90% during idling. That’s nearly double the baseline rate. At highway speeds the penalty shrinks to about 25%, because the engine is already working hard and the compressor load is a smaller fraction of total output. But at idle, the compressor becomes the dominant energy demand.
If your truck normally idles at 0.5 gallons per hour, flipping the AC on could push that to nearly a gallon per hour. Heating is less costly in most cases because many trucks use engine coolant heat, which requires no extra fuel, though diesel-fired heaters or electric heaters running off the alternator will add some load.
The CO2 Cost of Idling
A heavy-duty diesel truck produces roughly 10 pounds of carbon dioxide for every hour it idles. Research published in the Journal of the Air and Waste Management Association measured CO2 emissions of about 4,500 to 4,600 grams per hour (roughly 10.1 pounds) from idling heavy-duty diesel vehicles, regardless of fuel injection type. That’s the equivalent of driving about 7 to 8 miles down the highway, except the truck is going nowhere.
On top of CO2, idling produces nitrogen oxides, particulate matter, and carbon monoxide. These are the pollutants that prompted many states and cities to pass anti-idling laws, typically limiting commercial truck idling to 3 or 5 minutes in urban areas.
Restart vs. Idle: The 10-Second Rule
A common concern is whether restarting the engine wastes more fuel than just leaving it running. Research from Argonne National Laboratory settled this clearly: turning off the engine and restarting it uses less fuel than idling for as little as 10 seconds. Modern starters and fuel injection systems are efficient enough that the restart penalty is negligible. If you’re going to be stopped for more than 10 seconds and don’t need the engine running, shutting it off saves fuel every time.
This applies to all trucks, from pickups to semis. The old advice about starters wearing out from frequent restarts was more relevant decades ago. Modern starters are designed to handle tens of thousands of cycles.
Alternatives to Main-Engine Idling
For long-haul truckers who need climate control during rest periods, auxiliary power units (APUs) are the most common solution. These are small, separate engines mounted on the truck that power the heating, cooling, and electrical systems without running the main diesel engine. A typical diesel APU uses about 75% less fuel per hour than idling the tractor engine. If main-engine idling costs $20 a night, an APU brings that closer to $5.
Battery-powered electric APUs are also gaining traction, especially at truck stops that offer shore power connections. These eliminate fuel consumption entirely during rest periods, though they add upfront cost and weight. Some newer trucks come with idle-management systems that automatically shut the engine off after a set period and restart it only when cabin temperature drifts outside a preset range, splitting the difference between full idling and no idling at all.
Putting the Numbers Together
Here’s a quick reference for idle fuel consumption across truck types, with no accessories running:
- Class 8 semi-truck (diesel): 0.6 to 1.5 gallons per hour
- Medium heavy truck (gasoline, 5-7L): ~0.84 gallons per hour
- Delivery truck (diesel): ~0.84 gallons per hour
- Tow truck (diesel): ~0.59 gallons per hour
- Medium heavy truck (diesel, 6-10L): ~0.44 gallons per hour
Add up to 90% more if the air conditioning is running. For a typical pickup truck with a V6 or small V8, expect somewhere in the 0.3 to 0.5 gallon per hour range at idle with no load, scaling up with engine size and accessory use.