Diesel vs Petrol Fleet CO₂: How Speed Limiters Impact Each Fuel Type
Diesel vs Petrol Fleet CO₂: How Speed Limiters Impact Each Fuel Type
When a fleet manager installs speed limiters, the carbon saving achieved depends not only on how much speed is reduced — it also depends significantly on what fuel the vehicles burn. Diesel and petrol produce different amounts of CO₂ per litre combusted, and speed limiters affect different vehicle types by different percentages. Understanding this distinction is essential for prioritising which parts of your fleet to address first, and for accurately reporting your carbon savings in your ESG report.
This analysis breaks down the diesel vs petrol CO₂ comparison, examines how speed limiters affect each fuel type, and identifies which fleets generate the greatest carbon reduction from speed management.
The CO₂ Factors: Diesel vs Petrol
The fundamental difference begins at the pump. According to DEFRA’s 2024 GHG Conversion Factors — the authoritative source used for UK Scope 1 carbon reporting — the CO₂ produced per litre combusted is:
| Fuel Type | CO₂ per litre | Source |
|---|---|---|
| Diesel | 2.68 kg CO₂/litre | DEFRA GHG Conversion Factors 2024 |
| Petrol | 2.31 kg CO₂/litre | DEFRA GHG Conversion Factors 2024 |
Diesel produces approximately 16% more CO₂ per litre than petrol. This is because diesel has a higher carbon content per unit volume.
Why Diesel Is Still More Efficient
Despite its higher CO₂ per litre, diesel remains prevalent in commercial fleets because of its superior energy density. A diesel engine converts more of the energy in each litre into motion, which is why diesel vehicles typically achieve higher MPG (miles per gallon) figures than equivalent petrol vehicles. For high-mileage commercial operations — HGVs, vans, company cars covering 20,000+ miles per year — diesel’s fuel efficiency advantage has historically outweighed its higher CO₂/litre value.
However, this also means that diesel vehicles consume more litres per mile and emit more carbon per litre when they do burn fuel. Speed limiters, by reducing consumption, therefore deliver larger absolute CO₂ savings on diesel fleets than on equivalent petrol fleets, all else being equal.
Speed Limiter Impact by Fuel Type and Vehicle Category
The percentage fuel saving from a speed limiter is primarily determined by the degree of speed reduction and the aerodynamic drag characteristics of the vehicle — not directly by fuel type. However, when you combine the saving percentage with the fuel’s CO₂ factor, the carbon outcome differs.
| Vehicle Type | Fuel | Speed Reduction | Fuel Saving % | CO₂ Factor (kg/L) | CO₂ Impact |
|---|---|---|---|---|---|
| HGV Artic | Diesel | 56 → 52 mph | 10% | 2.68 | Large absolute saving |
| Van / LCV | Diesel | 70 → 60 mph | 12% | 2.68 | Medium saving, large fleet numbers |
| Van / LCV | Petrol | 70 → 60 mph | 12% | 2.31 | Slightly lower per litre |
| Company Car | Diesel | 80+ → 70 mph | 28% | 2.68 | High % × high CO₂/L |
| Company Car | Petrol | 80+ → 70 mph | 28% | 2.31 | Highest % saving, lower CO₂/L |
| Bus / Coach | Diesel | Governed | 11% | 2.68 | Significant at fleet scale |
The critical insight is that company cars show the largest percentage fuel saving (28%) because they are typically the most sensitive to speed reduction — motorway speeds above 70 mph impose disproportionately high aerodynamic drag penalties on cars. HGVs show a smaller percentage saving because they are already governed and operate within a narrower speed band, but their sheer fuel volume means the absolute CO₂ savings are enormous.
Worked Comparison: 10 Diesel Vans vs 10 Petrol Company Cars
To make this concrete, consider two fleet segments, each with 10 vehicles and the same annual mileage of 12,000 miles.
Scenario A: 10 Diesel Vans (35 MPG)
| Step | Calculation | Result |
|---|---|---|
| Baseline litres/van | (12,000 ÷ 35) × 4.54609 | 1,558.9 L |
| Litres saved/van (12%) | 1,558.9 × 0.12 | 187.1 L |
| Fleet litres saved | 187.1 × 10 | 1,871 L |
| CO₂ saved | 1,871 × 2.68 kg | 5.01 tCO₂e |
| Cost saved (£1.45/L) | 1,871 × £1.45 | £2,713/year |
Scenario B: 10 Petrol Company Cars (36 MPG)
| Step | Calculation | Result |
|---|---|---|
| Baseline litres/car | (12,000 ÷ 36) × 4.54609 | 1,515.4 L |
| Litres saved/car (28%) | 1,515.4 × 0.28 | 424.3 L |
| Fleet litres saved | 424.3 × 10 | 4,243 L |
| CO₂ saved | 4,243 × 2.31 kg | 9.80 tCO₂e |
| Cost saved (£1.42/L) | 4,243 × £1.42 | £6,025/year |
The Insight
Ten petrol company cars save nearly twice the CO₂ as ten diesel vans, despite a lower CO₂/litre factor. The reason is the dramatically higher speed reduction percentage (28% vs 12%) — petrol company cars are operating at speeds well above the optimal efficiency range, and the speed limiter captures those efficiency gains.
From a fleet carbon strategy perspective, this means company car fleets are often the highest-priority target for speed limiters, even though they use petrol (lower CO₂/litre), because the combination of high mileage, high-speed driving, and a large fleet count creates an outsized carbon opportunity.
Which Fleets Benefit Most from Speed Limiters for CO₂ Reduction?
Ranked by typical carbon reduction potential per vehicle:
1. Company Car Fleets (Motorway-Heavy Routes)
- 28% fuel saving — the highest of any category
- Even with petrol’s lower CO₂/litre, the large % saving produces substantial tCO₂e
- Very large fleet numbers in most organisations (often 50–500 vehicles)
- Also includes diesel company cars, which multiply both the % saving AND the CO₂/litre factor
2. HGV Articulated Lorries
- Only 10% fuel saving, but baseline consumption is enormous (30,000–50,000 litres per truck per year)
- Diesel with 2.68 kg CO₂/litre
- A single HGV articulated lorry saves approximately 4,000–5,000 litres of diesel per year from speed limiting — equivalent to 10.7–13.4 tCO₂e per truck
3. Van and LCV Fleets
- 12% saving — moderate, but the single most common fleet category in the UK
- Most vans are diesel, so the full 2.68 kg/litre factor applies
- Fleet numbers are typically large, making the aggregate saving significant
Comparison Table: CO₂ Savings Per Vehicle (Annual, 12,000 miles)
| Vehicle Type | Fuel | MPG | Baseline Litres | Saving % | Litres Saved | tCO₂e Saved |
|---|---|---|---|---|---|---|
| Company car | Petrol | 36 | 1,515 | 28% | 424 | 0.98 |
| Company car | Diesel | 45 | 1,213 | 28% | 340 | 0.91 |
| Van / LCV | Diesel | 35 | 1,559 | 12% | 187 | 0.50 |
| HGV artic (80K miles) | Diesel | 9 | 40,410 | 10% | 4,041 | 10.83 |
Hybrid and Electric Vehicles: A Different Calculation
Speed limiters remain relevant for hybrid and electric vehicles, but the CO₂ calculation differs significantly.
Mild and full hybrids (HEV): These vehicles still burn petrol or diesel during much of their operation, particularly at motorway speeds. Speed limiters reduce combustion engine use time and can improve EV mode penetration at lower speeds. The CO₂ savings are real but harder to calculate precisely without telematics data; a reasonable estimate is 40–60% of the equivalent pure-combustion saving.
Plug-in hybrids (PHEV): With sufficient charge, PHEVs may operate in electric-only mode on urban sections. At motorway speeds, the combustion engine is typically active. Speed limiters are still beneficial, but the carbon saving depends heavily on the vehicle’s charge management.
Battery electric vehicles (BEV): Speed limiters do not save combustion CO₂ because there is none. However, speed reduction does reduce electricity consumption (and therefore grid carbon emissions), and also reduces tyre and brake wear — both of which have embedded carbon in their manufacture. For fleet carbon reporting purposes, the impact of speed on BEV efficiency should be captured through Scope 2 (electricity) calculation rather than Scope 1.
For net zero strategies, fleets transitioning to EVs should not deprioritise speed limiters on their residual combustion fleet — the remaining diesel and petrol vehicles are likely to be the highest-mileage, hardest-to-electrify units, making speed management even more impactful during the transition.
Reporting Mixed Diesel and Petrol Fleets in Your ESG Report
For fleets with both diesel and petrol vehicles, calculate each fuel type separately and report the combined total.
Recommended ESG report statement:
“Fleet speed management interventions (speed limiter installation) delivered the following Scope 1 CO₂ reductions in [year]:
- Diesel fleet: [X] tCO₂e saved ([N] litres diesel avoided × 2.68 kg/litre, DEFRA 2024)
- Petrol fleet: [Y] tCO₂e saved ([M] litres petrol avoided × 2.31 kg/litre, DEFRA 2024)
- Combined: [X+Y] tCO₂e total reduction, representing a [Z]% reduction against the prior year baseline.”
This separate disclosure is best practice and aligns with DEFRA’s fuel-based Scope 1 methodology. Do not blend the CO₂/litre factors across fuel types — this is a common error that will understate diesel savings and overstate petrol savings or vice versa.
Practical Implications for Net Zero Strategy
Speed limiter decisions should be made in the context of your overall net zero roadmap:
- Prioritise company cars first if you have a large mixed fleet — the 28% saving per vehicle delivers the fastest ROI and the highest per-vehicle CO₂ reduction.
- Address HGVs for absolute tonnage — each truck saves 10+ tCO₂e per year. Even a small HGV fleet of 10 trucks delivers 100+ tonnes of annual CO₂ reduction.
- Van fleets deliver aggregate scale — 100 diesel vans with speed limiters save approximately 50 tCO₂e per year.
- Track your fuel split — if you are transitioning from diesel to petrol or HEV, your CO₂/litre assumption changes. Maintain separate records.
- Do not ignore the EV transition tail — during fleet electrification, the combustion vehicles that remain are often the highest-emitting units. Maximise speed management on them while EV rollout proceeds.
Use the Fleet Savings Calculator
Use our free Fleet Savings Calculator to calculate your exact CO₂ and fuel savings. Try the calculator →
The calculator handles both diesel and petrol vehicles and supports mixed fleet configurations. Enter your fleet data to receive a breakdown by fuel type, suitable for inclusion in your ESG report.
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