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The average miles per gallon of a semi-truck is bad enough when barreling down the highway — somewhere in the range of 6-8 MPG — but things don’t look much better when the 18-wheeler’s at a standstill. Even when it’s not moving, an idling semi-truck continues to burn fuel at an alarming rate. It explains why idling the engine while stopped has become such an expensive habit in the trucking industry: Whether it’s to keep power going to the cabin appliances or to keep things feeling comfortable while loading up, idling is far from an innocent act. It racks up fuel costs, damages engines, and harms the environment to boot.

Just how much fuel does it burn? On average, a long-haul semi burns about 0.8 gallons of diesel per hour while idling. This number can vary depending on engine size, load, and fuel type, with some trucks going through as much as 1.5 gallons per hour while idle. Over the course of a year, one can imagine this adds up pretty quickly. A typical long-haul truck idles around 1,800 hours annually, which translates to burning approximately 1,500 gallons of diesel over that year-long period. Doing the math (and accounting for an average diesel cost of about $3.70 a gallon), that comes out to be nearly $6,000 in unnecessary fuel costs — and that’s just one year and just one truck. For an entire fleet year-over-year, that number’s only going to go up.

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Wasting fuel while idle doesn’t just drain the driver’s wallet… It puts financial strain on entire fleets, even if they don’t realize it. A trucking company with hundreds of vehicles idling regularly could lose hundreds of thousands of dollars annually to fuel costs alone. When you consider that fuel makes up a major portion of a fleet’s operational budget, cutting down on unnecessary idling could be a major money-saver. And yet, the practice remains widespread due to driver habits and perceived convenience.

Fuel isn’t the only resource being burned while a truck idles. Engine wear and tear becomes a lot more rampant under idle conditions. Because the semi’s inline-6 engine isn’t operating at its optimal temperature, fuel doesn’t fully combust. This causes carbon to build up in the cylinders. Over time, this buildup damages internal parts and can cause as much as twice the wear compared to driving at normal speeds.

It’s estimated that excessive idling can drive up maintenance costs by $2,000 per vehicle per year. Add that to the unnecessary fuel costs, and the already huge bill gets even bigger. Not to mention the environmental cost: semi-truck idling emits 11 million tons of carbon dioxide, 55,000 tons of nitrogen oxides, and 400 tons of particulate matter annually. That’s in the U.S. alone.

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Thanks to the financial, mechanical, and environmental harm of idling, many states have put anti-idling laws into place with fines reaching up to $25,000. California, New Jersey, New York, and Pennsylvania all enforce idling limits under 10 minutes (with specific exemptions for certain weather or operational needs). Some states take it even further: in certain cases, semi-truck idling is banned completely in Wyoming.

Semi-truck drivers can cut down on idling with the help of an Auxiliary Power Unit (APU). This is a device that gives power to all the trucks heating, cooling, and electronics without a need for the main engine to be running. While they can cost anywhere from $8,500 to $13,000 to purchase and install, they would theoretically pay for themselves in less than three years based on our math above.

GPS fleet tracking systems can also help fleets monitor drivers’ idle time. These systems collect data on when and where trucks idle, giving managers a chance to see who’s doing what and correct the bad habit accordingly. Small adjustments like these could ultimately make a big difference (even in the semi-trucks with the best MPG).