The short answer
Anticipate and coast rather than accelerating and braking, keep the tyres at the right pressure, take the roof bars off, and slow down on the motorway.
- Look far ahead and lift off early. Every time you brake you are throwing away fuel you already paid for.
- Correct tyre pressures. Underinflation costs several per cent and wears the tyres out faster at the same time.
- Take the roof box and bars off when not in use. The aerodynamic penalty is large and constant.
- Slow down. Drag rises with the square of speed, so 80mph uses substantially more than 70mph for a small time saving.
- Remove the weight you are carrying around for no reason.
- Use the highest sensible gear without labouring the engine.
What will ruin it
Do not coast in neutral. Modern engines cut fuel entirely when you lift off in gear, whereas in neutral they must idle, so coasting out of gear uses more fuel and removes engine braking.
Why it works
Fuel is spent overcoming drag, rolling resistance and the inertia of accelerating a tonne of metal. Only the third of those is recoverable by driving style, but it is the largest in town driving.
Tipking is general household information, not professional advice. Test anything on a hidden patch first. Full disclaimer.