The UK is one of the few countries still using miles for road distances and miles per hour for speed, despite broader European metrication. This creates a somewhat mixed system for UK drivers, where fuel is sold in litres but road signs show miles.
Road Tax and CO2 — A Metric System
While distance measurements remain imperial, the core vehicle tax system is entirely metric. CO2 emissions are measured in grams per kilometre (g/km), even in the UK. Engine sizes are measured in litres. Fuel consumption — though rarely tested in real-world conditions — uses litres per 100 kilometres. The VED bands are expressed in g/km CO2, making the road tax system itself fully metric regardless of the UK's general approach to distance measurement.
Distance-Based Road Tax Proposals
Several proposals for road pricing reform have suggested a distance-based tax — charging drivers per mile travelled rather than an annual lump sum. These proposals would require a metric of miles or kilometres for each journey. GPS-based systems or black box telematics could track mileage. The UK has trialled voluntary distance-based insurance policies, but a full national road pricing scheme remains under discussion. If introduced, it would use miles as the standard unit — maintaining the UK's imperial measurement tradition in motoring.
EU and Metrication Changes
The UK is no longer subject to EU rules requiring metrication of road signs and distances. Post-Brexit, the government has confirmed that imperial measurements — miles on signs, mph for speed — will remain. The UK was required to display supplementary metric distances on signs during EU membership, but these supplementary signs have been allowed to remain as they are already in place. The mixed system — miles on roads, litres at pumps, g/km for tax — reflects a pragmatic approach to measurement rather than a fully coherent policy.
Fuel Economy and MPG
Fuel economy in the UK is commonly expressed in miles per gallon (MPG), though gallons themselves are not a standard international measurement. The US gallon differs from the UK (imperial) gallon, creating confusion for international comparisons. The official EU measure for fuel economy — litres per 100km (L/100km) — is more informative and is increasingly used in car specifications. Road tax calculations use CO2 g/km, which is consistent across all measurement systems, but drivers often think in MPG when estimating fuel costs.
What Drivers Need to Know
For road tax purposes, the measurement system is straightforward: CO2 in g/km determines your VED band, and this is measured and recorded in the same way across all UK vehicles. Distance-based road pricing, if introduced, would use the mile as the unit. For everyday driving, the mixed imperial-metric system means drivers should be comfortable converting between MPG and L/100km when comparing vehicle efficiency — particularly when looking at official fuel economy figures or comparing UK-spec cars against international models.