A heavy-duty sleeper with the engine idling burns roughly 0.8 gallons an hour, and closer to 1.0 to 1.2 gal/hr once the A/C is loaded or the engine is working against a high-idle setting. That is not a rounding error. A truck idling 6 hours a day, 250 days a year burns about 1,200 gallons doing zero work. At $3.60 a gallon that is roughly $4,300 per truck per year, and at today's diesel price it is closer to $5,500. The fixes cost between $200 and $12,000, and the payback math is different for each one.
What each type of truck burns while it sits
Idle burn is mostly a function of engine displacement and what accessories are pulling on it. These are the working numbers most fleets and engine makers use. Treat them as planning figures, then confirm against your own telematics, because a truck sitting in a 95-degree parking lot with the A/C maxed will run well above the base rate.
- Heavy-duty sleeper (Class 8, 12 to 15L). About 0.8 gal/hr at base idle. 1.0 to 1.2 gal/hr with A/C or hotel loads, and higher again on a fast-idle setting.
- Tank wagon or bobtail (Class 7 to 8, medium diesel). Roughly 0.5 to 0.8 gal/hr. Add PTO work on top: a product pump running off the PTO typically pushes the engine to 0.9 to 1.5 gal/hr depending on rpm and pump load.
- Medium-duty box or service truck (Class 5 to 6). Around 0.4 to 0.6 gal/hr.
- Gasoline light-duty (pickup, van). Roughly 0.2 to 0.5 gal/hr, higher on a big V8 with the A/C on.
The one that surprises people is the tank wagon. Drivers leave the truck running through an entire drop because the pump needs the PTO, and that time gets logged as idle even though the truck is doing real work. Before you write a policy, split PTO idle from parked idle in your reporting. If you are running mobile and onsite fueling, PTO hours are simply the cost of doing business and belong in your delivered price, not in your idle-abuse column.
The dollars on one truck, worked out
Take a single sleeper. Six hours of idle a day, 250 working days a year, at 0.8 gal/hr:
6 hrs x 250 days x 0.8 gal = 1,200 gallons a year. At $3.60/gal that is $4,320. At the current U.S. average diesel price of $4.58/gal (as of July 2026, per FuelDataPortal), the same 1,200 gallons cost $5,496. Diesel is down 12.1% over the last 30 days, which softens the number a little, but it does not change the shape of the decision. Multiply by twenty trucks and you are looking at $86,000 to $110,000 a year burned standing still.
Turning a telematics idle percentage into a dollar figure
Most telematics platforms report idle as a percentage of engine-on time rather than as hours. The conversion is simple. Take annual engine hours, multiply by the idle percentage, multiply by your burn rate, multiply by your cost per gallon. A truck with 2,400 engine hours a year running 35% idle is idling 840 hours, burning 672 gallons, costing about $3,080 at $4.58. Do that for every unit and sort descending. In practice, three or four trucks usually account for a third of the total, and those drivers are the entire conversation. This is one of the few operating numbers worth tracking every month alongside the handful of numbers that actually tell you how the business is doing.
The three fixes and what each one pays back
There are really only three levers, and they scale in cost and in what they solve.
1. Idle shutdown timers. Effectively free. Every modern engine ECM (Cummins, Detroit, Paccar, Volvo) has a programmable idle shutdown, usually settable from 3 to 15 minutes, with optional overrides for extreme ambient temperature or low battery. Your dealer can set it during a PM visit. Set it to 5 minutes and you kill the bulk of casual idling with no capital outlay. It does nothing for the driver who has to sleep in the truck in January, which is why it is a starting point rather than the whole program.
2. Bunk heaters. An Espar (now Eberspächer) Airtronic or a Webasto Air Top runs $1,500 to $3,500 installed and burns roughly 0.05 gal/hr, drawing fuel from the main tank. Against 0.8 gal/hr of main-engine idle, you are saving about 0.75 gal every hour it runs. A driver who used to idle 6 hours a night for heat, 120 nights a year, was burning 576 gallons ($2,600 at $4.58). The heater burns 36 gallons instead. Payback under one year on a cold-weather truck. This is the best dollar-for-dollar fix in the entire category. Its limit is obvious: it gives you heat, not air conditioning.
3. APUs. A diesel auxiliary power unit (Thermo King TriPac, Carrier ComfortPro, Rigmaster) runs $9,000 to $12,000 installed and burns about 0.2 gal/hr while delivering heat, A/C, and shore power for the bunk. Against 0.8 gal/hr, you save 0.6 gal for every hour the main engine would have run. A truck idling 1,200 hours a year saves about 720 gallons, or roughly $3,300 at $4.58. On a $10,500 install that is a payback of about 3.2 years on fuel alone, and it comes in sooner if diesel runs high or the truck idles more than average. Battery-electric APUs remove the fuel burn entirely but cost the same or more, carry battery replacement cost every 3 to 5 years, and run out of A/C capacity on a long hot night.
The decision rule most fleets land on: timers on everything, bunk heaters on any truck that sleeps north of the Mason-Dixon line, and APUs only on sleepers that idle more than about 1,000 hours a year and that you plan to keep at least four more years. If the truck is going to auction in eighteen months, the APU does not pay you back, though it does help resale.
The part that is not fuel: idle hours are engine miles
Fuel is the visible cost. The hidden one is wear. The figure fleets actually quote, drawn from the Argonne National Laboratory and EPA idle-reduction work and repeated in fleet-maintenance guidance, is that one hour of idling puts on roughly 25 to 33 miles worth of engine wear. Engine makers do not publish a miles-per-idle-hour conversion of their own, so treat this as a planning multiplier rather than a warranty term. What the OEMs do instead is write PM intervals against engine hours or fuel burned as well as odometer miles, which is the same idea arrived at from the other direction.
Run that against the truck above. Twelve hundred idle hours a year is somewhere around 30,000 to 40,000 equivalent miles of wear that never showed up on the odometer. A truck you think has 100,000 miles on it is wearing like a truck with 130,000 or more, and if you are setting oil and filter intervals off the odometer alone you are under-servicing it by a wide margin. Idling at low rpm also runs the engine cold and rich, which loads up the diesel particulate filter and drives more frequent regens. Long-idling trucks are the ones that come back with aftertreatment faults, and a DPF replacement out of warranty is a $3,000 to $5,000 line item all by itself.
Policy, and the reason drivers idle
Hardware fixes the cause you can buy your way out of. A policy fixes the rest. The three habits worth attacking, in order of dollars: leaving the truck running through a 40-minute lunch, leaving it running at a customer site during paperwork, and leaving it running overnight in a yard where shore power exists. All three are behavior, and all three are visible in the telematics.
Publish a monthly idle-percent ranking by driver, tie a small bonus to it, and be honest about the exclusions. Do not penalize a driver for PTO hours or for cab heat when it is 8 degrees out. Fleets that skip that step get gaming instead of savings, because drivers will simply unplug the tracker or find an override. Also check your states: many, including California and New Jersey, cap unnecessary idling at 3 to 5 minutes with real fines, and the enforcement is not evenly applied but it is not theoretical either.
The hardest part is usually not the analysis. It is getting fuel gallons, engine hours, and driver assignment into the same place at the same time, because those three things typically live in three different systems. That is exactly the seam a real jobber back office is supposed to close, and if you are pulling a spreadsheet from your cardlock network, another from your telematics, and a third from payroll every month, you already know how that goes. FastDragon exists partly because that reconciliation should not take a person three days.
Start with the timer setting. It costs nothing, it can be done at the next PM, and it will tell you within a month how much of your idle was habit and how much was necessity. Then spend the capital on the trucks the data points at.
Questions people ask
Does adding an APU cost me payload capacity?
Federal law gives you room for it. Under 23 U.S.C. 127, as amended by the FAST Act, a vehicle equipped with qualifying idle-reduction technology can exceed the 80,000 lb gross limit by the certified weight of the unit, up to 550 pounds. You have to be able to prove the weight and demonstrate the unit is functional at roadside, so keep the manufacturer's certification in the cab.
Is fuel burned by an APU or bunk heater eligible for any tax treatment?
It is diesel drawn from the main tank, so it was taxed at the pump like any other on-road gallon, and there is no separate refund path for it. That differs from PTO fuel, where a number of states do allow a partial refund or exemption for the portion of fuel used to run equipment rather than propel the vehicle. The rules and the substantiation requirements vary by state, so check with your state's motor fuel tax division before you claim anything.
How do I keep drivers from disabling the idle shutdown?
Most ECMs let you lock the parameter so it can only be changed with dealer software, and they log override events. Enable the ambient-temperature and battery-voltage overrides so the shutdown does not strand a driver in genuine cold, then review the override report monthly. If one truck shows fifty overrides and the rest show three, you have a conversation, not a hardware problem.
Do electric APUs make sense for a fleet that mostly runs regional?
Often yes, because regional trucks tend to sit for shorter overnight stretches and get back to a yard with shore power. Battery units have no fuel burn and no fuel filter or oil service, which is a real maintenance saving. The catch is battery replacement every three to five years and limited A/C runtime on a hot night, so they fit better in temperate regions and shorter rest periods than in a Texas summer over a 10-hour break.
How much of my telematics idle number is actually PTO time?
On a tank wagon it is frequently half or more. If your platform supports a PTO input wire, get it connected during the next install cycle so PTO engagement is logged separately. Without it you are looking at one blended idle percentage and you will end up disciplining drivers for doing their job.