Short trips kill UK fuel economy: the 5-mile rule, with city-driving evidence

Why cold engines drink fuel

A modern petrol or diesel engine is optimised for one operating temperature: fully warm, with coolant near 90°C and the catalyst lit off. Until it gets there, the engine control unit deliberately runs a richer air-fuel mixture to keep combustion stable and to heat the catalyst quickly. Friction is also higher because oil is thicker when cold, and ancillary loads (heated screen, blower, lights) are typically at their peak in the first few minutes.

The DVSA's own guidance for the official MPG test acknowledges this: the WLTP cycle begins with a cold start precisely because it is the worst-case for emissions and consumption. Independent testing by the AA and by consumer titles has repeatedly shown that the first mile or two of any journey can return well under half of a car's steady-state MPG figure.

In practice, that means a car quoted at 55 MPG combined might be doing 20–25 MPG for the first mile from cold. Stretch that trip to ten or fifteen miles and the average recovers. Do only two miles and park up? You never leave the penalty zone.

The 5-mile rule, and what UK trip data shows

The rough rule of thumb used by fleet trainers and driving instructors is simple: below about five miles, expect materially worse MPG than the official figure; below two miles, expect roughly half. It is not a law of physics, but it aligns with what ONS travel data and manufacturer cold-start curves imply.

The National Travel Survey (DfT) has consistently shown that a large share of car trips in England are under five miles, and roughly a quarter are under two miles. These are exactly the journeys where the engine never fully warms, the catalyst spends longest lighting off, and the MPG penalty is heaviest. It is also where walking, cycling or combining errands has the biggest financial return.

The chart below sketches the pattern: MPG rises sharply with trip length before flattening out once the powertrain is at temperature. The exact curve varies by car, ambient temperature and driving style, but the shape is remarkably consistent across independent tests.

What it costs at UK pump prices

Take a driver doing 8,000 miles a year, half of it in short urban hops. If those short trips average 28 MPG instead of the car's 50 MPG potential, the extra fuel burn is roughly 60–70 litres a year. At recent DESNZ weekly petrol prices around 135 p/L, that is £80–£95 spent purely on cold-running losses.

For a diesel the gap is narrower in percentage terms (diesels warm more slowly but run leaner cold), but absolute pence-per-mile costs are similar because diesel is priced a few pence higher at the pump. RAC Fuel Watch and the AA Fuel Price Report both publish these weekly averages if you want to sense-check your own numbers.

None of this makes short driving irrational — sometimes you have to make the trip. But it does mean the marginal cost of a two-mile car journey is much higher than a simple 'pence per mile' calculation based on the official MPG suggests.

Practical ways to blunt the penalty

You cannot cheat thermodynamics, but you can reduce how often you pay the cold-start tax. The biggest lever is trip-chaining: combining several errands into one longer outing so the engine only warms up once.

A few habits also help on the trips you do make:

The bottom line for UK drivers

Short trips are the single most under-appreciated fuel drain in UK motoring. They rarely show up in advertised MPG figures, they are heavily represented in DfT travel statistics, and at current pump prices they quietly add up to real money.

If you are trying to squeeze more MPG out of your existing car, the highest-return change is usually not a gadget or an additive. It is restructuring the shortest one or two journeys in your week — either by combining them, or by not making them by car at all.