The fuel economy figure on a car's official specification sheet is measured on a laboratory dynamometer using a standardised drive cycle — not on a road — and the gap between official figures and real-world consumption has been so consistently wide that regulators developed the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) specifically to close it, with only partial success
The previous articles on this site covered MPG and L/100km unit basics, EV efficiency vs petrol costs, hypermiling techniques, the MPG illusion (10→20 saving more than 30→50), and why lab figures differ from real-world driving. This article addresses fuel economy measurement standards — specifically how NEDC, WLTP, and the EPA test cycle differ, what variables they each fail to capture, and what this means for comparing vehicles across markets.
The three major test cycles and their histories
NEDC (New European Drive Cycle): introduced in the 1970s, updated in 1997. A standardised speed profile lasting 20 minutes that included four Urban Driving Cycles (speeds up to 50 km/h) and one Extra Urban Driving Cycle (speeds up to 120 km/h). NEDC was designed for the 1970s traffic conditions and vehicle performance levels — low speeds, low accelerations, long idling periods.
The NEDC problem: by the 2000s and 2010s, the test was being gamed. Manufacturers discovered that small optimisations — taping over door seams to reduce drag, over-inflating test tyres, using thinner lubricants, pre-conditioning the battery — produced substantially better NEDC figures without improving real-world performance. By 2015, the average gap between NEDC figures and real-world consumption was approximately 40%.
WLTP (Worldwide Harmonised Light Vehicle Test Procedure): introduced in the EU from September 2017, mandatory from September 2018. Designed with more aggressive accelerations, higher average and maximum speeds (131 km/h), a longer test duration (30 minutes), and tighter conditions that make optimisation harder:
- Higher average speed: 46.5 km/h vs NEDC's 33.6 km/h
- Higher maximum speed: 131 km/h vs NEDC's 120 km/h
- Longer test: 30 minutes vs NEDC's 20 minutes
- More dynamic drive profile: less constant-speed, more acceleration and deceleration
WLTP also introduced per-vehicle testing based on optional equipment: a car with larger wheels and more aerodynamic drag is tested under conditions that reflect its actual specifications — preventing manufacturers from testing a stripped-out base model and applying those figures to optioned vehicles.
EPA (Environmental Protection Agency, US): the US has used a different approach since 2008, testing fuel economy in laboratory conditions but applying "real-world adjustment factors" to account for the gap between lab and road. The EPA publishes adjusted figures (reflecting an estimated real-world experience of roughly 80-90% of lab performance) and provides separate city/highway/combined ratings.
The temperature variable: why cold weather destroys fuel economy
Fuel economy figures are tested at approximately 23°C — an issue because internal combustion engines and EV batteries both perform significantly worse in cold weather:
Petrol and diesel engines in cold weather:
- Engine oil is thicker when cold, increasing internal friction
- Fuel doesn't atomise as efficiently, requiring richer mixtures during warm-up
- More engine idling during cold start
- The catalytic converter needs to reach ~400°C to function — during warm-up, it's less effective and the ECU runs rich mixtures
The cold weather penalty: real-world fuel consumption in winter (ambient temperature 0°C) is typically 15-25% higher than in summer for the same driving profile.
EV batteries in cold weather:
- Lithium-ion cells have higher internal resistance at low temperatures, reducing efficiency
- Battery heating requires significant power before driving efficiency is available
- Regenerative braking is often limited in very cold conditions (battery management systems limit charge rate when cells are cold)
Real-world EV range at -10°C: typically 30-40% below the WLTP rated range. A car rated 400 km WLTP range might deliver 250-280 km in freezing conditions with heating in use.
What WLTP still doesn't capture
Despite improvements over NEDC, WLTP has remaining gaps:
Air conditioning: AC use is not included in the standard WLTP test. Real-world driving with AC on in summer increases fuel consumption by 5-15% for petrol cars and 15-25% for EVs (where cabin heating and cooling draw directly from the battery).
Cold start: the test begins with a warmed engine. Cold starts in real-world driving impose extra fuel consumption not captured in the figure.
High-speed motorway driving: WLTP's maximum speed of 131 km/h misses the fuel consumption at 140-160 km/h common on some European motorways and US highways. Aerodynamic drag scales with velocity squared — doubling speed quadruples drag.
Traffic patterns: real-world traffic varies by region, city, season, and time of day. No single test cycle can represent all driving contexts.
Comparing fuel economy across markets
Different markets use different units and different test protocols, making direct comparison difficult:
UK (post-WLTP): mpg (UK imperial gallon, 4.546 litres), WLTP figure US EPA: mpg (US gallon, 3.785 litres), EPA adjusted figure EU, Australia, most of the world: L/100km, WLTP figure
Why UK and US mpg figures for the same car differ even beyond the test cycle: the UK gallon is 20% larger than the US gallon. The same car achieving 40 mpg (UK) achieves approximately 33 mpg (US) — the same fuel consumption, different numbers.
Why EU and UK WLTP figures may differ for the same model: optional equipment (larger wheels, panoramic roofs, additional weight) affects the WLTP result. The EU and UK tests are conducted under the same protocol but for the specific variant combination — a lightly equipped model may achieve a different WLTP figure than the same model heavily optioned.
L/100km vs MPG: the inverse relationship and its consequence
L/100km and MPG are inverse of each other — higher L/100km means worse fuel economy, higher MPG means better fuel economy. This inversion creates an asymmetry in how improvements are perceived:
5 L/100km → 4 L/100km: saves 1 litre per 100 km 10 L/100km → 9 L/100km: saves 1 litre per 100 km (same absolute saving)
In MPG terms: 5 L/100km = 56.5 mpg (UK); 4 L/100km = 70.6 mpg (UK): a 14-point improvement for an already-efficient vehicle 10 L/100km = 28.2 mpg (UK); 9 L/100km = 31.4 mpg (UK): a 3.2-point improvement
The MPG illusion (covered in the previous article) emerges from this inverse relationship — improvements in L/100km are linear (equally valuable per litre saved), but improvements in MPG appear larger for already-efficient cars. L/100km is the more intuitive unit for representing absolute fuel saving.
How to use the Fuel Economy Converter on sadiqbd.com
- Cross-market comparison: convert UK mpg to L/100km when comparing UK and EU car reviews for the same model — the unit difference obscures whether the figures are actually different or just expressed differently
- UK vs US mpg: when a US car review mentions mpg, divide by 1.201 to get the approximate UK mpg equivalent (the UK gallon is 20.1% larger) — a US car rated 32 mpg would be approximately 38.4 mpg UK
- EV kWh/100km to equivalent L/100km: some comparisons express EV efficiency as a petrol-equivalent consumption — the converter provides a basis for this comparison, with petrol having approximately 33.7 kWh per gallon (US) or 8.8 kWh per litre
Frequently Asked Questions
Why do car manufacturers still advertise WLTP figures even though everyone knows they're optimistic? Because regulation requires it, it's the only standardised basis for comparison, and advertising real-world figures would require specifying the exact driving conditions — which vary infinitely. WLTP figures are a controlled, repeatable laboratory measurement that allows comparing two cars on the same basis. The fact that both cars' real-world consumption is higher than their WLTP figures doesn't undermine the comparative value — if Car A achieves 5 L/100km WLTP and Car B achieves 7 L/100km WLTP, Car A will almost certainly be more fuel-efficient in real-world use even if neither achieves its rated figure. The absolute WLTP figure is unreliable; the relative WLTP comparison between similarly-tested cars is still informative.
Is the Fuel Economy Converter free? Yes — completely free, no sign-up required.
Try the Fuel Economy Converter free at sadiqbd.com — convert between MPG (UK and US), L/100km, km/L, and kWh/100km instantly.
