Why does my car use less fuel on the highway than it does around town?

Fuel consumption is one of the things we most talk about when discussing our cars.

It's a no-brainer that most of us believe they use too much fuel, particularly around town where the consumption is much higher than it is on the highway or in the country.

If only we could get the same fuel economy in town as we get on the open road.

Unfortunately, there is a range of factors that prevent that from happening.

Different driving environments

The reason for the difference between city and country fuel consumption can mostly be found in the difference between the respective driving environments that dictate the way we drive.

In the city we tend to make shorter trips, to work, driving the kids to school, going to the supermarket and the mall etc.

We regularly have to stop, at traffic lights, stop and give-way signs, pedestrian crossings, the school, shops, for incidents on the road, and traffic jams.

Even when moving we're almost always driving in a transient mode, such as accelerating, decelerating and braking.

But in the country we tend to make longer journeys, don't stop as much, and that means we're able to run more at a steady speed.

Cold not cool

Our engines aren't at their most efficient when warming up from a cold start and fuel consumption during that phase is higher than it is once the engine is warmed-up.

To prevent our engines from stalling, and stumbling etc. and generally being unpleasant and unsafe to drive during the warm-up phase they're fed with a richer fuel mixture.

With the short trips we make around town our engines spend a greater percentage of our trip times cold or warming up.

Some cars used only for short local trips can spend an inordinate amount of time in this partly warmed-up, inefficient state.

Idling going nowhere

For many reasons we're often forced to come to a halt in city traffic, and whenever we do our engine returns to idle. Idling consumes fuel while we go nowhere.

When we're driving in the country, particularly on freeways or multi-lane highways, we very rarely have to stop, so we don't spend anywhere nearly as much time idling.

Idle fuel consumption is such a large part of our city fuel consumption that carmakers have developed fuel-saving devices like stop/start that shuts the engine down whenever we stop and automatically restarts it again when we move away.

Getting going

The laws of physics say that getting a mass moving requires more effort than keeping it rolling.

As we're regularly accelerating, after stopping or even just keeping pace with the ever-changing speed of traffic our cars have to work harder in town than they have to on the highway where we rarely have to accelerate.

Geared to go

Carmakers generally gear our cars to operate in the efficient part of the rev range in top gear at typical highway speeds, whereas in the city the engines are running at varying speed, not always the most efficient.

CVT transmissions are one attempt to keep the engine running at a constant, fuel-efficient speed while the gearing continuously changes to suit the circumstances.

Carmakers are also moving to automatic transmissions with a greater number of gears to reduce the sometimes large gaps between gears in older transmissions, so the engine is running more often in its most efficient range.

Hybrids are the exception

While it holds true for most cars that they will get lower fuel consumption running on the highway than they will around town there are exceptions to the rule.

Hybrids, for instance, do similar mileage or better around town to that they do on the highway.

The reason for that is that the hybrid drive system, a combination of electric motor and small combustion engine, is tailored to city driving when most of the running is done on the electric motor, and the petrol engine only kicks in to assist when needed.

Hybrids also employ intelligent systems to harness energy, such as that generated by braking, when it would otherwise be wasted.

ADR fuel consumption

The government requires carmakers to declare the fuel consumption for their cars and the results of their testing, for urban (city), extra-urban (country) and combined (average) driving, are posted on the stickers they attach to the windscreen of their new cars.

The fuel consumption figures are determined in the sanitised environment of a test laboratory rather than real life and are intended for comparison purposes only.

Related: Fuel efficiency ratings – what do they tell you?