How to calculate rental car emissions
Ground road transport — rental cars, employee-owned vehicles used for business, and company cars — is a consistent source of Scope 3 Category 6 emissions across most enterprise travel programmes. While typically lower-volume than aviation, road transport emissions can be significant for field sales teams, client-facing roles and organisations that operate in areas with limited public transport. This guide covers what falls under Category 6 for road travel, how to calculate it accurately, and how electric rental cars change the picture.
What counts as Category 6 road travel?
The GHG Protocol Scope 3 Category 6 includes employee business travel by personal or rented vehicle. This covers:
- Rental cars: cars hired specifically for a business trip, regardless of fuel type.
- Employee mileage claims: journeys made in personally-owned vehicles for which the employee claims a business mileage allowance or expense.
- Company vehicles used for business travel: where emissions are not already captured in Scope 1. If the company owns the vehicle and pays for fuel directly, those emissions are typically Scope 1. If an employee drives a company car and expenses fuel, it depends on company policy and accounting approach.
- Taxi: Taxi rides, such as airport transfers, are counted separately to this type of road travel. → How to calculate taxi and rideshare emissions
Not included in Category 6: daily commuting by car is Scope 3 Category 7 (Employee Commuting).
How to calculate road travel emissions
Road travel uses the same method as other modes:
| Activity Data distance, fuel, or spend | × | Emission Factor kg CO₂e per km, litre, or £ | = | CO₂e (kg) your figure |
Method 1: Spend-based
Where only expenditure data is available, a spend-based approach using EEIO factors is possible as a fallback, but is significantly less accurate than distance-based and not recommended for reporting where trip-level booking data can be sourced from a TMC or expense system.
| How Thrust Carbon differs Thrust Carbon applies a regression analysis based on over 18,000 car rentals in the Thrust Carbon system that have pricing data attached, producing a kgCO₂e per $ (or equivalent currency) figure. |
Method 2: Distance-based (recommended default)
The activity data is the distance driven in kilometres. The emission factor is expressed in kg CO₂e per vehicle kilometre and varies by vehicle category, size and fuel type. For rental cars, if the specific vehicle type is unknown, an average medium petrol car factor is used as the default.
For employee mileage claims, organisations can either use the distance in the mileage claim (if submitted) or estimate from postcode-to-postcode routing. Some tools integrate with expense systems to capture mileage data automatically where it is available.
Method 3: Fuel-based
The most accurate calculation, but fuel data is often unavailable. If rental car and road travel represents your most material travel emission category, you should try to use fuel data wherever available.
Emission factors by vehicle type
DEFRA publishes annual emission conversion factors for UK-based road transport.
| Vehicle type | DEFRA factor (kg CO₂e/km) | Example: 450 km journey |
|---|---|---|
| Small petrol car | 0.183 | 82 kg CO₂e |
| Medium petrol car | 0.224 | 101 kg CO₂e |
| Large petrol car | 0.343 | 154 kg CO₂e |
| Average diesel car | 0.215 | 97 kg CO₂e |
| Battery electric (UK grid) | 0.051 | 23 kg CO₂e |
| Hybrid petrol | ∼0.147 | 66 kg CO₂e |
Source: DEFRA 2025 Greenhouse Gas Conversion Factors. Figures include WTT (well-to-tank) emissions. UK grid electricity intensity used for EV calculation. Actual values vary by vehicle specification.
Electric vehicle rental cars: not zero-emission
Electric vehicle rental cars are significantly lower-carbon than petrol equivalents, but they are not zero-emission. The emissions come from the electricity used to charge them — and the carbon intensity of that electricity depends on the grid mix in the country and time of charging.
Best practice for accounting for electric vehicle rentals:
- Obtain quantity data (kWh) and apply country specific grid mix electricity emission factors.
- Obtain supplier specific distance emission factors (kgCO2e/km) based on the energy mix used by the supplier.
How to reduce rental car and road travel emissions
Ground transport emissions are highly actionable — the levers available to travel managers range from modal substitution to vehicle type policy and reimbursement rate design, each with a measurable impact on both emissions and cost.
🚆 Public transport substitution
The highest-leverage reduction opportunity for ground transport is switching to rail for intercity trips where available. For journeys under 500km, rail is typically both lower-cost and 70–90% lower carbon than a rental car. How to calculate rail travel emissions →
⚡ EV rental policy
In markets with clean grids (UK, France, Nordics), an EV rental policy meaningfully reduces road transport emissions for equivalent journeys. In markets with more fossil fuel reliant grids, the benefit is smaller — a nuanced policy may be more effective than a blanket EV mandate.
💷 Mileage rate design
Mileage reimbursement rates that differentiate by vehicle type — paying a lower pence-per-mile rate for larger, higher-emission vehicles — create a natural incentive toward smaller, more efficient cars without requiring mandates. Some organisations set a maximum reimbursable vehicle category for certain trip types.
Capture every rental car and road journey, automatically
Thrust Carbon integrates with 40+ expense and TMC systems to pull vehicle mileage data directly into your Scope 3 inventory. All modes, one platform.
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