Fuel consumption varies substantially with how and where you drive. The factors that matter are measurable and several popular tips are not among them.
What matters most
Speed. Air resistance rises with the square of speed, so consumption climbs sharply above about eighty kilometres per hour. The difference between a hundred and a hundred and twenty is larger than most people expect.
Acceleration and braking. Every acceleration converts fuel into speed and every brake application converts that speed into heat. Anticipating and coasting saves meaningfully.
Tyre pressure. Underinflation increases rolling resistance measurably and it is free to fix.
Short journeys. A cold engine runs rich and inefficiently. Journeys under a few kilometres consume disproportionately, which is why urban consumption is so much worse than the official figures.
What matters moderately
Weight and roof loads. A roof box costs a substantial percentage on the motorway, and leaving it fitted when unused is wasteful.
Air conditioning, which uses fuel. At high speed, open windows cost more through drag, so the advice reverses depending on speed.
What does not matter much
Premium fuel in a car that does not require it. If the manual specifies regular, higher octane provides no benefit.
Most fuel additives sold as economy improvers.
Idling to warm up a modern engine, which warms faster while driving gently.
The biggest single factor
Trip planning. Combining errands into one journey with a warm engine beats every driving technique.