Short answer
Fuel is the visible cost of idling and the smaller half of the story. Every idle hour also advances the engine-hour counter that manufacturers write service intervals against, while the odometer does not move at all. So the same hour is charged twice, and only one of the two charges appears in a fuel report.
Key takeaways
- US DOE/Argonne put long-haul truck idling at roughly 1,800 hours a year, burning about 1,500 gallons of diesel.
- DOE states explicitly that idling costs more where warranty and maintenance intervals apply to hours rather than miles.
- SAE J1939 already separates the idle share: SPN 235 Engine Total Idle Hours and SPN 236 Idle Fuel Used, both in PGN 65244.
- An odometer records nothing for a stationary hour, so a distance-based schedule cannot charge idling at all.
- Teltonika's Excessive Idling defaults, 5 s to stopped, 2 s to moving, decide what your idle total even contains.
01The bill everyone sees
Idling has a fuel cost, it is measurable, and it is usually the whole of the conversation. Argonne National Laboratory publishes per-hour burn rates by vehicle class: a Class 8 tractor-semitrailer with no accessory load consumes about 0.64 gallons per hour, roughly 2.42 litres, and the same tractor with air conditioning running consumes 1.15 gallons per hour, about 4.35 litres. A medium heavy truck sits at 0.44 gallons and a Class 5 delivery truck between 0.84 and 1.11 depending on load. Argonne's own analysis puts diesel burn at approximately one gallon per hour with a 5 brake-horsepower accessory load at 1,000 rpm. Multiply any of those by an annual idle figure and the number is large. But it is only the part of the cost that leaves through the fuel tank.
02The bill nobody budgets for
The second cost is that an idle hour is a full hour of engine life. US DOE and Argonne estimate long-haul truck idling at approximately 1,800 hours per year, consuming about 1,500 gallons of diesel, and DOE states explicitly that this matters as a warranty and maintenance-interval consequence: where intervals apply to hours rather than miles, idling costs more. That sentence is the whole argument. Eighteen hundred hours is not a rounding error against a service schedule measured in hundreds of hours, it is several complete intervals arriving a year earlier than a distance-based plan expected. The oil, the filters, the labour and the downtime all get consumed by hours the truck spent standing still. Nothing in a fuel report shows that, because the expenditure is not fuel.
03Why the odometer cannot charge for it
The reason this cost hides so well is structural rather than accidental. SAE J1939 defines Total Vehicle Distance, SPN 245, at 0.125 kilometres per bit, and it advances when the wheels turn. A stationary hour produces no distance, so a distance-based service rule charges nothing for it, not a small amount, but exactly zero. Engine hours behave the opposite way: SPN 247, Engine Total Hours of Operation, accumulates at 0.05 hours per bit purely from running time, capturing idling, power-take-off use and every other stationary engine operation. Two counters, one situation, and completely different bills. A fleet that schedules on distance is therefore not underestimating idling wear; it has no mechanism to see it, and the more idle-heavy the duty cycle the further the schedule drifts from the engine's actual condition.
04The standard already isolates the idle share
You do not have to infer the idle portion, because the vehicle is willing to report it directly. J1939 defines SPN 235, Engine Total Idle Hours, at a resolution of 0.05 hours per bit, and SPN 236, Engine Total Idle Fuel Used, at 0.5 litres per bit, both carried in PGN 65244. That the standard bothers to separate idle hours from total hours, and idle fuel from total fuel, is itself the industry's acknowledgement that idling is a distinct cost category rather than a subset to be ignored. For a fleet with a CAN adapter fitted, this changes the reporting question from an estimate to a read: how many of this engine's accumulated hours were idle, and how many litres went with them. That is the figure to set against the service interval, and the vehicle has been counting it all along.
05What counts as idling depends on two small numbers
Before comparing idle totals between vehicles or months, it is worth knowing that the total is defined by configuration. Teltonika's Excessive Idling scenario has an entry debounce, parameter 11205 "Time to stopped", defaulting to 5 seconds across a range of 0 to 3,600, and an exit debounce, parameter 11206 "Time to moving", defaulting to 2 seconds across the same range. The documented entry condition is duration-gated rather than instantaneous: speed must stay below the start-speed threshold for the whole timeout before the state is entered. The asymmetry is deliberate, it takes longer to declare an idle than to end one, and the scenario ships disabled by default. Change either value and the idle hours in your report change with it, which is why an unexplained month-on-month jump should send you to the configuration before the drivers.
06Air conditioning is not the exception people assume
In Gulf conditions the standard defence of idling is cab cooling, and the data does not make it free. Argonne's figures show accessory load nearly doubling the Class 8 burn rate, from 0.64 to 1.15 gallons per hour, and the official fueleconomy.gov site run by DOE and EPA states that air-conditioning use can cut fuel economy by more than 25% in very hot conditions. Meanwhile the engine-hour counter is indifferent to the reason, an hour idled for comfort advances SPN 247 exactly as far as an hour idled in a queue. So the honest framing for a hot-climate fleet is not that idling should be eliminated, which is neither safe nor realistic, but that cab cooling has a measured price in fuel and an unmeasured one in service intervals, and both belong in the decision about how the waiting time is managed.
07Charging idling to the right account
The remedy is an accounting change more than a behavioural campaign. First, bind every hour-based service rule to the engine-hour counter rather than to distance or to ignition-on time, so idle hours are charged where they are actually spent. Second, where a CAN adapter exists, pull SPN 235 and SPN 236 and report idle hours and idle litres as their own line rather than folding them into totals. Third, record the Excessive Idling configuration alongside the numbers, so a comparison between two vehicles is a comparison of behaviour and not of settings. Fourth, when presenting the cost of idling to an operations team, show both halves, the litres and the fraction of the next service interval consumed. The second figure is usually the one that changes a decision, because it has a date attached.
| Counter or account | What one idle hour adds | Can it see idling? |
|---|---|---|
| Odometer / SPN 245 distance | Nothing | No, structurally blind |
| Engine hours / SPN 247 | A full hour | Yes |
| Idle hours / SPN 235 | A full hour, isolated | Yes, and separately |
| Idle fuel / SPN 236 | 0.5 L per bit resolution | Yes, and separately |
| Fuel account, Class 8 no load | ≈0.64 gal (≈2.42 L) | Yes |
| Fuel account, Class 8 with AC | ≈1.15 gal (≈4.35 L) | Yes |
| Hour-based service interval | One hour of the budget | Yes, DOE names this |
| Distance-based service interval | Nothing | No |
Idling fuel use by truck type and accessory load
Argonne National Laboratory, Idling Reduction Savings Calculator
Figures and sources
- Long-haul truck annual idling hours and diesel used, per US DOE/Argonne: ≈1,800 h/yr; ≈1,500 gallons diesel (Source: US DOE Clean Cities / Argonne National Laboratory, Long-Haul Truck Idling, as of 2026-08-31)
- DOE/Argonne statement that idling costs more where intervals apply to hours rather than miles: stated explicitly as a warranty/maintenance-interval consequence (Source: US DOE Clean Cities / Argonne National Laboratory, as of 2026-08-31)
- Idling fuel use, Class 8 tractor-semitrailer, with and without accessory load: 0.64 gal/h (≈2.42 L/h) no load; 1.15 gal/h (≈4.35 L/h) with air conditioning (Source: Argonne National Laboratory, Idling Reduction Savings Calculator, as of 2026-08-31)
- Idling fuel use, medium heavy truck and Class 5 delivery truck: 0.44 gal/h medium heavy; 0.84 gal/h no load and 1.11 gal/h with load for Class 5 (Source: Argonne National Laboratory, Idling Reduction Savings Calculator, as of 2026-08-31)
- Argonne finding on diesel idling fuel burn with cab heating or AC operating: ≈1 gal/h at 5 bhp accessory load and 1,000 rpm (Source: Argonne National Laboratory, ANL/ESD-43, as of 2026-08-31)
- DOE/EPA figure for how much AC use can cut fuel economy in very hot conditions: more than 25% (Source: fueleconomy.gov (US DOE / US EPA official site), as of 2026-10-18)
- SAE J1939 SPN 235, Engine Total Idle Hours, and SPN 236, Engine Total Idle Fuel Used, resolutions: 0.05 hr/bit and 0.5 L/bit, both in PGN 65244 (Source: SAE J1939-71 (Revised MAR2011), Vehicle Application Layer, as of 2026-08-31)
- SAE J1939 SPN 247 engine hours versus SPN 245 distance, the two counters an idle hour affects differently: SPN 247 at 0.05 hr/bit advances; SPN 245 at 0.125 km/bit does not (Source: SAE J1939-71 (Revised MAR2011), Vehicle Application Layer, as of 2026-08-31)
- Teltonika Excessive Idling entry and exit debounce, parameters, defaults and ranges: Parameter 11205 "Time to stopped" default 5 s; parameter 11206 "Time to moving" default 2 s; both 0–3,600 s (Source: Teltonika Telematics Wiki, FMB120 Parameter list, as of 2026-08-31)
- Documented entry condition for the idling state, duration-gated, and shipped disabled: speed must stay below the start-speed threshold for the whole Time to stopped timeout; scenario default disabled (Source: Teltonika Telematics Wiki, FMC003 Parameter list, as of 2026-08-31)
Questions
How much does idling really cost beyond fuel?
It costs a full hour of engine life per idle hour. At the DOE/Argonne long-haul figure of roughly 1,800 idle hours a year, that is several complete hour-based service intervals arriving earlier than a distance-based plan expected, oil, filters, labour and downtime. DOE names hour-based intervals as exactly where idling costs most.
Can I read idle hours directly instead of estimating them?
Yes, where a CAN adapter is fitted. SAE J1939 defines SPN 235 Engine Total Idle Hours at 0.05 hours per bit and SPN 236 Engine Total Idle Fuel Used at 0.5 litres per bit, both in PGN 65244. The vehicle has been counting both all along; the adapter is what lets you read them.
Should drivers just switch the engine off while waiting?
Not as a blanket rule in Gulf heat, where cab cooling is a safety matter. The point is that the cooling has a measured price, accessory load nearly doubles the Class 8 burn rate, and an unmeasured one in service intervals. Both belong in how waiting time is planned, not in a slogan.
Why did my idle hours change without driver behaviour changing?
Check the configuration first. Teltonika's Excessive Idling entry debounce defaults to 5 seconds and the exit debounce to 2 seconds, both adjustable from 0 to 3,600, and the scenario ships disabled. Either value changes what the total contains, so two months are only comparable if the settings match.