Why Average Emission Factors Fail HGV Fleets — and What to Use Instead

Most carbon accounting software for logistics works the same way: multiply distance travelled by an emission factor (expressed in kgCO2e per tonne-kilometre), and report the result. The GLEC Framework and ISO 14083 both permit this approach as a baseline method.

The problem is that average emission factors are, by definition, averages. For a heavy goods vehicle operating in variable real-world conditions, the gap between the average and the actual can be substantial.

What Average Emission Factors Miss

A standard HGV diesel emission factor is derived from aggregate data across many vehicles, routes, and operating conditions. It does not account for:

Load variation. A vehicle carrying 28 tonnes burns significantly more fuel per kilometre than the same vehicle carrying 12 tonnes on the same route. Emission factors typically assume an average load factor.

Gradient. A route across the Pennines demands substantially more fuel than an equivalent-distance motorway run across the Midlands. Gradient is not captured in a distance-based emission factor.

Vehicle-specific efficiency. Two HGVs of the same nominal type will have different engine efficiencies, tyre rolling resistance values, and aerodynamic profiles — all of which affect actual fuel consumption.

Driving behaviour. Aggressive acceleration and braking patterns increase fuel consumption above the modelled average. Fuel-efficient driving reduces it.

The cumulative effect of these variables means that a fixed emission factor applied to a diverse HGV fleet will produce CO2e estimates that are accurate on average but systematically wrong at the individual journey level.

The Hardware Alternative — and Its Limits

Portable Emissions Measurement Systems (PEMS) are the regulatory gold standard for vehicle emissions measurement. A PEMS unit installed on an HGV measures exhaust gases directly, producing accurate real-world emissions data.

The limitation is economic. A PEMS unit costs tens of thousands of pounds and requires specialist operation. It is used for type-approval testing and periodic compliance verification — not for continuous monitoring across an operational fleet.

A Physics-Based Software Approach

An alternative to both average emission factors and physical sensors is software that models the vehicle's actual fuel consumption from its telemetry data — applying the physical equations that govern vehicle motion to derive what the engine must have consumed on a given journey.

This approach uses inputs that most modern HGVs already generate: GPS speed data, route elevation, and fuel card records. From these, the governing equations of longitudinal vehicle dynamics can reconstruct fuel consumption at a level of precision that generic emission factors cannot approach.

The output is CO2e data specific to each journey — calculated from the physics of that particular vehicle, on that particular route, under that particular load — rather than derived from population-level averages.

This is the calculation methodology EcoTrace is developing: software that applies the same physical principles as hardware PEMS, without requiring hardware installation on every vehicle.