Mobile Combustion — Own Fleet Methodology and Calculation Approach
Every litre of fuel an organisation burns in a vehicle it owns or controls is a Scope 1 emission — the exhaust of the cars, vans, trucks, ships, aircraft, and off-road plant that move a business around. The accounting looks simple: multiply fuel by a factor. But two questions decide whether the number is right. Do you have fuel records, or only distance? And are you using a factor built for a moving engine, or one built for a boiler?
Mobile combustion is where fuel data quality meets tailpipe chemistry. Get the method and the factor right and the inventory is defensible; reach for the convenient shortcut and it quietly drifts.
Own-fleet mobile combustion is a Scope 1 emission, calculated either fuel-based (fuel consumed × an emission factor for that fuel) or, where fuel records are missing, distance-based (kilometres × a per-kilometre vehicle factor). The fuel-based method is preferred; both use DEFRA factors on an AR5 GWP-100 basis.
This page is the citation-grade treatment of own-fleet mobile combustion — how the direct emissions of vehicles and mobile plant an organisation owns or controls are quantified as Scope 1. It covers the two calculation methods and when each applies, the fuel emission factors and their four unit bases, the per-kilometre distance-based fallback, why a moving engine emits a different methane and nitrous-oxide profile than a stationary boiler burning the same fuel, the global warming potential basis, and the well-to-tank emissions that sit in Scope 3 rather than Scope 1. The factors are DEFRA 2026 conversion factors reported under the GHG Protocol, on an IPCC AR5 GWP-100 basis. It pairs with the Scope 1 mobile combustion calculator and sits alongside the diesel, LPG, and natural-gas combustion methodologies and the well-to-tank methodology.
What Mobile Combustion Is
Mobile combustion is the burning of fuel in vehicles and mobile equipment — as distinct from stationary combustion, which is fuel burnt in fixed equipment like boilers, furnaces, and generators. When the asset is owned or controlled by the reporting organisation, its mobile combustion is a Scope 1 direct emission.
The fleet that generates mobile combustion is broad: road vehicles (company cars, vans, trucks), marine vessels, aircraft, and off-road and specialist plant such as forklifts, agricultural machinery, and construction equipment. What unites them is that the organisation both controls the asset and buys the fuel, so the tailpipe emissions belong in its own Scope 1 inventory rather than in a transport supplier’s.
The boundary that puts mobile combustion in Scope 1 is operational or financial control of the asset, combined with responsibility for the fuel. A vehicle the organisation owns and fuels is Scope 1. A courier or haulier the organisation hires to move goods is a purchased transport service — Scope 3 Category 4 (upstream) or Category 9 (downstream) — even though the same diesel is being burned. The fuel is identical; the boundary is what differs.
This methodology covers the Scope 1 case: the organisation’s own fleet, its own fuel. Outsourced transport is accounted separately under the relevant Scope 3 category using the same underlying factors reinterpreted at the service boundary.
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The Two Methods: Fuel-Based vs Distance-Based
There are two ways to quantify mobile combustion, and the choice is driven by what activity data the organisation actually has.
| Dimension | Fuel-based | Distance-based |
|---|---|---|
| Activity data | Fuel consumed (litres, tonnes, kWh) | Distance travelled (km) |
| Factor | Per-unit-fuel emission factor | Per-km vehicle factor |
| Accuracy | Higher — measures actual fuel burnt | Lower — assumes typical fuel economy |
| Data source | Fuel cards, purchase records, tank dips | Odometer, telematics, mileage claims |
| When to use | Whenever fuel records exist (preferred) | Fallback when fuel data is unavailable |
Use the fuel-based method whenever fuel records exist. It measures the actual fuel burnt, so it captures real-world load, terrain, driving style, and vehicle condition — everything a per-kilometre average smooths over. The distance-based method is a fallback for when fuel data is genuinely unavailable: it multiplies distance by an assumed fuel economy baked into the vehicle factor, which is a reasonable estimate but not a measurement. Never treat the two as interchangeable inputs to the same total.
The two methods are not additive — a given vehicle’s emissions are counted once, by whichever method its data supports. Mixing them across a fleet is fine (fuel-based for vehicles with fuel cards, distance-based for those without), but double-counting a single vehicle by both routes is a common and serious error (§11).
The Fuel-Based Method
The fuel-based method multiplies the quantity of each fuel consumed by its emission factor. It is the primary method because the fuel burnt is the physical driver of the emission, and fuel records — from fuel cards, bulk purchase invoices, or tank measurements — are usually the most reliable activity data a fleet has.
The headline diesel factor renders live above. The fuel factors come in four unit bases, and the right one depends on how the fuel consumption is recorded.
Match the unit basis to the fuel record. Use the per-litre basis for forecourt liquid fuels and most road and marine fleet calculations; the per-tonne basis for solid or bulk gaseous fuel deliveries measured by mass; and the two per-kWh bases (net or gross calorific value) where the fuel consumption is already expressed in energy units. Net and gross calorific value differ by roughly the latent heat of the water vapour in the exhaust, so mixing them, or pairing a net-CV activity figure with a gross-CV factor, introduces a systematic error of several percent.
Cite the mobile factor, not the stationary one. The mobile-combustion factors are read with [gc_factor key="mobile_combustion.<fuel>.<unit>"] — they carry the Scope 1 mobile-combustion activity tag. The generic fuel shortcode resolves the stationary-combustion row instead: the same numeric value, but tagged as stationary, which is the wrong activity classification for a vehicle. For a fleet inventory, always resolve the mobile-combustion keyspace explicitly.
Fuel Emission Factors
The mobile-combustion keyspace carries fifteen fuels, each on the four unit bases. The per-litre factors for the liquid and liquefied-gas fuels are shown below; all render live from the DEFRA 2026 data layer.
| Fuel | kg CO₂e per litre (live) |
|---|---|
| Diesel (average biofuel blend) | 2.58354 |
| Diesel (100% mineral) | 2.66155 |
| Petrol (average biofuel blend) | 2.075 |
| Petrol (100% mineral) | 2.35372 |
| Gas oil (red diesel) | 2.75541 |
| Aviation spirit (avgas) | 2.33116 |
| Aviation turbine fuel (Jet A/A-1) | 2.54269 |
| Marine gas oil | 2.77139 |
| Marine fuel oil (residual) | 3.10202 |
| Fuel oil (residual) | 3.17492 |
| LPG | 1.55713 |
| Propane | 1.54358 |
| Butane | 1.74533 |
| Compressed natural gas (CNG) | 0.43885 |
| Liquefied natural gas (LNG) | 1.14791 |
Every value renders live from the DEFRA 2026 mobile-combustion keyspace on an AR5 GWP-100 basis. Each fuel is also available on per-tonne and per-kWh (net or gross calorific value) bases for consumption recorded by mass or energy. The gaseous fuels (CNG, LNG) are more commonly metered by mass or energy than by litre.
The values are identical to the stationary-combustion factors for the same fuels — the same fuel releases the same carbon whether burnt in an engine or a boiler — but they carry the mobile-combustion activity classification, which is what a fleet inventory needs for correct categorisation and for the tailpipe methane and nitrous-oxide treatment discussed in §6.
The Distance-Based Method
When fuel records are unavailable — a pool of grey-fleet vehicles reimbursed by mileage, say, or a vehicle whose fuel-card data is incomplete — the distance-based method estimates emissions from kilometres travelled and a per-kilometre factor specific to the vehicle type. The factor embeds an assumed fuel economy, so the result is an estimate rather than a measurement.
| Vehicle | kg CO₂e per km (live) |
|---|---|
| Car — average, diesel | 0.17265 |
| Car — dual-purpose 4×4, diesel | 0.19794 |
| Motorbike — small, petrol | 0.08319 |
| Van — average, diesel | 0.25716 |
| Articulated HGV — all, average laden, diesel | 0.93374 |
Per-kilometre factors render live from the DEFRA 2026 passenger-vehicle and delivery-vehicle keyspaces. Cars are classified by DEFRA size (small/medium/large/average) or by segment (mini through luxury, plus 4×4, MPV); heavy goods vehicles by size and laden weight; both on an AR5 GWP-100 basis, tail-pipe (tank-to-wheel) only. Electric vehicles are not mobile combustion — their charging is Scope 2 purchased electricity.
Pick the vehicle classification that matches your fleet knowledge. DEFRA offers cars by both size class and market segment; use whichever your fleet records support, and the average row where the vehicle detail is unknown. For heavy goods vehicles, the laden factor matters — an empty return leg emits far less per kilometre than a full outbound one, and the average-laden row is the right default only when actual loading is unknown.
Per-kilometre factors are not per-litre factors — never multiply distance by a fuel factor or fuel by a distance factor. The two keyspaces answer different questions: distance × per-km vehicle factor, or fuel × per-unit-fuel factor. Crossing them (kilometres × a per-litre fuel factor, for instance) produces a meaningless number off by orders of magnitude. And a single vehicle is counted by one method only, never both.
The Three Gases: CO₂, CH₄, N₂O
Burning transport fuel emits three greenhouse gases, and their relative importance differs between a moving engine and a stationary boiler burning the same fuel. Carbon dioxide dominates in both. But mobile combustion produces a proportionally larger share of methane and nitrous oxide, because engines run through cold starts, catalyst warm-up periods, and constant load transients that a steady-state boiler avoids.
Mobile combustion emits the same fuel’s carbon but a different methane and nitrous-oxide profile than stationary combustion. Cold starts before the catalytic converter reaches operating temperature, and the transient loads of real-world driving, raise the CH₄ and N₂O share relative to a steady-state boiler. The DEFRA mobile-combustion factor already accounts for this — which is why the correct activity classification matters even though the headline CO₂e value coincides with the stationary factor. Nitrous oxide is a particular concern because its very high GWP magnifies a small mass into a meaningful CO₂-equivalent.
The published emission factor is a single CO₂-equivalent value that already sums the three gases at their respective global warming potentials. Where a per-gas breakdown is needed — for regulatory reporting that itemises CO₂, CH₄, and N₂O separately — the factor decomposes into its carbon-dioxide, methane, and nitrous-oxide components, each of which is a documented sub-field of the factor record rather than a separately published headline value.
Global Warming Potential and CO₂e
The three gases are combined into a single carbon-dioxide-equivalent figure using each gas’s global warming potential — the factor that expresses a mass of methane or nitrous oxide as the mass of CO₂ that would cause the same warming over a chosen time horizon.
The DEFRA mobile-combustion factors are published on an AR5 GWP-100 basis. This is the assessment-report basis DEFRA uses for its regulatory conversion factors, and it is the basis on which the live CO₂e values on this page are expressed. An organisation reporting under a regime that mandates a different GWP basis — a newer assessment report, or a different time horizon — must convert consistently and disclose which basis it used. Do not silently blend factors published on different bases within one inventory total.
For most fleet reporting, the DEFRA AR5 basis is applied as published and stated in the inventory’s methodology notes. The distinction becomes material only when reconciling against another source published on a different basis, or when a disclosure regime specifies its own required GWP set — in which case the per-gas decomposition (§6) allows re-conversion of the methane and nitrous-oxide components at the required values.
Well-to-Tank: The Scope 3 Companion
The fuel factors on this page are tank-to-wheel — the emissions from actually burning the fuel in the vehicle. But every litre of fuel also carries an upstream burden from extracting, refining, and distributing it before it reaches the tank. That well-to-tank portion is real, but it does not belong in Scope 1.
Well-to-tank emissions are Scope 3, not Scope 1. The combustion of the fuel in an owned vehicle is Scope 1; the upstream extraction, refining, and distribution of that fuel is Scope 3 Category 3 (fuel- and energy-related activities not included in Scope 1 or 2). They are reported in different scopes and must not be added into the Scope 1 mobile-combustion total. The well-to-tank factor is a companion to the combustion factor, read from the fuel keyspace with a well-to-tank suffix, and reported in its own line.
A complete fuel footprint reports both: the tank-to-wheel combustion emission in Scope 1, and its well-to-tank companion in Scope 3 Category 3. The well-to-tank methodology covers the upstream side and the well-to-tank concept in full; the point for a Scope 1 fleet inventory is simply to keep the two scopes separate.
Calculation Formulas and Data Sources
| Quantity | Formula |
|---|---|
| Fuel-based emission | Σ (fuel consumed × fuel emission factor) |
| Distance-based emission | Σ (distance travelled × per-km vehicle factor) |
| Fleet Scope 1 total | Σ (fuel-based vehicles) + Σ (distance-based vehicles) |
| Well-to-tank (Scope 3 Cat 3) | Σ (fuel consumed × WTT factor) — reported separately |
Data sources
| Input | Source | How it enters |
|---|---|---|
| Fuel emission factors (15 fuels × 4 bases) | DEFRA 2026 | Live — [gc_factor key="mobile_combustion.*"] |
| Per-km vehicle factors | DEFRA 2026 | Live — [gc_factor key="passenger_vehicles.*" / "delivery_vehicles.*"] |
| Per-gas CO₂/CH₄/N₂O components | DEFRA 2026 | Factor sub-fields (audit record) |
| Well-to-tank factors | DEFRA 2026 | Live — [gc_factor key="fuels.gbr.<fuel>.wtt.<unit>"] |
| GWP basis | IPCC AR5 GWP-100 (DEFRA convention) | Applied in the published factor |
| Fuel consumed / distance travelled | Fleet records | User-supplied activity data |
The factors on this page — fuel factors, per-km vehicle factors, and well-to-tank companions — all render live from the DEFRA 2026 data layer. The activity data — how much fuel each vehicle burnt, or how far it travelled — comes from the organisation’s own fleet records. The quality of a mobile-combustion inventory is set almost entirely by the quality of that activity data, not by the factors.
Worked Example — A Mixed Fleet (Audit Record)
A single hardcoded audit record: one organisation’s fleet for a reporting year, mixing fuel-based and distance-based vehicles. All values are illustrative and hardcoded — they reconcile to their stated inputs and represent no real organisation. Factors are the DEFRA 2026 AR5 GWP-100 values shown live elsewhere on this page.
| Vehicle group (illustrative inputs) | Method | tCO₂e |
|---|---|---|
| Van fleet: 42,000 litres diesel (avg biofuel), × 2.58354 | Fuel-based | 108.5 |
| Company cars: 18,000 litres petrol (avg biofuel), × 2.075 | Fuel-based | 37.4 |
| Forklifts: 6,500 litres LPG, × 1.55713 | Fuel-based | 10.1 |
| Grey fleet (no fuel data): 120,000 km, avg diesel car × 0.17304 | Distance-based | 20.8 |
| Fleet Scope 1 total | 176.8 | |
| Well-to-tank companion (reported in Scope 3 Cat 3) | Fuel-based | ≈ 41.2 |
Two features are characteristic. First, the three fuel-based groups dominate the Scope 1 total, with the distance-based grey fleet a smaller and less certain contribution — the estimate that carries the most uncertainty is the one built on assumed rather than measured fuel use. Second, the well-to-tank companion is a substantial figure in its own right (roughly a quarter of the tank-to-wheel total), and it sits entirely outside the Scope 1 line — reporting it inside would overstate Scope 1 by that amount. Each vehicle group is counted once, by a single method, and the fuel-based and distance-based subtotals are summed only after each is computed on its own basis.
Edge Cases and Known Pitfalls
Don’t double-count a vehicle by both methods
Each vehicle is counted once — fuel-based where fuel records exist, distance-based where they don’t. Running a vehicle through both routes and summing them double-counts its emissions. Reconcile the fleet list against both data sources and assign each vehicle a single method.
Cite the mobile factor, not the stationary one
The mobile-combustion factors carry the correct Scope 1 mobile activity tag; the generic fuel shortcode resolves the stationary row with the same value but the wrong classification. For a fleet inventory, resolve the mobile-combustion keyspace explicitly so the activity categorisation and tailpipe gas treatment are right.
Keep well-to-tank out of Scope 1
The upstream extraction and refining of fuel is Scope 3 Category 3, not Scope 1. Adding the well-to-tank companion into the Scope 1 mobile-combustion total overstates Scope 1 by roughly a quarter. Report tank-to-wheel in Scope 1 and well-to-tank in its own Scope 3 line.
Electric vehicles are not mobile combustion
A battery-electric vehicle burns no fuel, so it has no Scope 1 mobile-combustion emission. Its charging is purchased electricity — Scope 2. Plug-in hybrids split between the two. Putting an EV’s charging into the mobile-combustion line misclassifies a Scope 2 emission as Scope 1.
Don’t cross the unit bases
Fuel factors come per litre, per tonne, and per kWh (net or gross calorific value). Pair each activity figure with the matching basis. Multiplying a litre figure by a per-tonne factor, or a net-CV consumption by a gross-CV factor, introduces an error from a few percent to orders of magnitude.
Match the laden factor to the load
Heavy goods vehicle per-km factors vary sharply with load — an empty return leg emits far less per kilometre than a full outbound one. Use the laden band that reflects actual loading; the average-laden row is the right default only when real loading is unknown.
Red diesel is its own fuel
Gas oil (red diesel), used in off-road and agricultural plant, has its own factor distinct from road diesel. Applying the road-diesel factor to red-diesel consumption, or vice versa, misstates the emission. Match the fuel factor to the actual fuel dispensed.
State the GWP basis
DEFRA factors are AR5 GWP-100. If a disclosure regime requires a different assessment-report basis, convert the methane and nitrous-oxide components consistently and disclose the basis used. Blending factors from different GWP bases within one total makes the inventory non-reproducible.
What the Calculator Handles vs What You Decide
A calculator automates the factor lookups, the unit handling, and the scope separation. It cannot supply the fleet’s activity data or decide which vehicles belong in which method.
The calculator handles
Looking up the correct DEFRA mobile-combustion factor for each fuel and unit basis; applying per-km vehicle factors for the distance-based method; keeping the four unit bases consistent; computing the tank-to-wheel Scope 1 total; and computing the well-to-tank companion in Scope 3 Category 3 as a separate line rather than folding it into Scope 1.
You decide
The fleet inventory — which vehicles are owned or controlled; each vehicle’s fuel consumption or distance; which method each vehicle’s data supports; the vehicle classification and laden band for the distance-based rows; whether a vehicle is combustion, electric, or hybrid; and the GWP basis if a regime requires one other than DEFRA’s AR5. The tool computes; the fleet boundary and activity data are yours.
Build a Scope 1 mobile-combustion inventory for an owned fleet — fuel-based and distance-based, with DEFRA 2026 factors, correct unit handling, and the well-to-tank companion kept in Scope 3.
Standards and Data Sources
| Source | Role for this methodology |
|---|---|
| GHG Protocol Corporate Standard | The inventory boundary under which owned-fleet mobile combustion is reported as a Scope 1 direct emission, and the scope definitions that place well-to-tank and outsourced transport in Scope 3. |
| GHG Protocol Scope 1 combustion guidance | The Scope 1 combustion accounting basis, from which mobile combustion is distinguished by asset mobility and its tailpipe methane and nitrous-oxide profile. |
| DEFRA 2026 conversion factors | The source of every fuel factor, per-km vehicle factor, and well-to-tank companion used on this page, on an AR5 GWP-100 basis. |
| IPCC AR6 | The reference for global warming potential values; DEFRA applies AR5 GWP-100 for its regulatory factors, with AR6 relevant where a regime requires re-conversion. |
| Well-to-tank methodology | The companion methodology for the upstream Scope 3 Category 3 emissions of the same fuel. |
| Diesel · LPG · Natural gas combustion | The per-fuel combustion methodologies for the fuels a fleet burns, covering each fuel’s factor derivation in detail. |
Frequently Asked Questions
Mobile combustion is the burning of fuel in vehicles and mobile equipment — road vehicles, ships, aircraft, and off-road plant — as opposed to stationary combustion in fixed equipment like boilers. When the organisation owns or controls the asset and buys the fuel, its mobile combustion is a Scope 1 direct emission.
The fuel-based method multiplies fuel consumed by an emission factor and is preferred because it measures the actual fuel burnt. The distance-based method multiplies kilometres travelled by a per-kilometre vehicle factor and is a fallback for when fuel records are unavailable, since it assumes a typical fuel economy rather than measuring consumption. Each vehicle is counted by one method only.
The same fuel releases the same carbon dioxide, so the headline CO₂e values coincide. But a moving engine runs through cold starts, catalyst warm-up, and load transients that raise the methane and nitrous-oxide share relative to a steady-state boiler. The DEFRA mobile-combustion factor accounts for this profile, which is why the correct mobile activity classification matters even where the value matches the stationary factor.
No. A battery-electric vehicle burns no fuel and has no Scope 1 mobile-combustion emission; its charging is purchased electricity, reported under Scope 2. Plug-in hybrids split between mobile combustion for their fuel and Scope 2 for their charging. Classifying an EV’s charging as mobile combustion misplaces a Scope 2 emission in Scope 1.
No. Burning the fuel in an owned vehicle is Scope 1 (tank-to-wheel); the upstream extraction, refining, and distribution of that fuel is Scope 3 Category 3 (well-to-tank). They are reported in different scopes. Adding the well-to-tank companion into the Scope 1 mobile-combustion total overstates Scope 1 by roughly a quarter.
Match it to how the fuel is recorded. Use per-litre for forecourt liquid fuels and most road and marine calculations, per-tonne for solid or bulk gaseous fuels measured by mass, and per-kWh (net or gross calorific value) where consumption is already in energy units. Pairing a net-calorific-value activity figure with a gross-calorific-value factor, or crossing the bases, introduces a systematic error.
No. If the organisation hires a courier or haulier rather than running its own vehicle, that is a purchased transport service — Scope 3 Category 4 (upstream) or Category 9 (downstream) — even though the same diesel is burned. Mobile combustion in Scope 1 requires the organisation to control the asset and buy the fuel.
Carbon dioxide, methane, and nitrous oxide. Carbon dioxide dominates, but mobile combustion produces a proportionally larger methane and nitrous-oxide share than stationary combustion because of cold starts and transient loads. Nitrous oxide matters disproportionately because its high global warming potential turns a small mass into a meaningful CO₂-equivalent. The published factor sums all three.
The DEFRA mobile-combustion factors are published on an AR5 GWP-100 basis, DEFRA’s convention for its regulatory conversion factors, and that is the basis of the CO₂e values here. An organisation reporting under a regime that mandates a different assessment-report basis must convert the methane and nitrous-oxide components consistently and disclose which basis it used.
Gas oil (red diesel), used in off-road, agricultural, and construction plant, has its own emission factor distinct from road diesel, and it is Scope 1 mobile combustion when burnt in owned mobile equipment. Apply the red-diesel factor to red-diesel consumption; using the road-diesel factor instead misstates the emission.