Fuel is the running cost drivers watch most closely, which has produced a large body of advice of wildly varying quality.
What genuinely works
Slowing down on the highway. Aerodynamic drag rises with the square of speed, so the energy needed to push through the air climbs steeply. Most vehicles are meaningfully less efficient at 80 mph than at 65, and this is the largest single lever available to most drivers.
Smoother driving. Hard acceleration followed by hard braking converts fuel into heat at the brake discs. Anticipating traffic and coasting toward stops preserves momentum you already paid for.
Correct tire pressure. Underinflated tires increase rolling resistance. The effect per tire is small but it is free, and it also extends tire life.
Removing weight and drag. A roof box or roof rack left on permanently costs a noticeable amount at highway speed, because it disrupts airflow where the vehicle is otherwise slippery. Take it off between trips. Weight in the trunk matters less than people think, but it is not nothing.
Combining trips. A cold engine is markedly less efficient until it reaches operating temperature. Several short separate trips use considerably more fuel than one longer trip covering the same distance.
What makes little or no difference
Premium fuel in an engine that does not require it. Higher octane resists knock; it contains no more energy. If the manual says regular, premium buys nothing. If it says premium required, use it. If it says premium recommended, the engine will adapt to regular with a small loss of power and efficiency, and the arithmetic is usually close to neutral.
Fuel additives and economy devices. Magnets, vortex generators, fuel line gadgets and most bottled additives do not produce measurable gains in a normally functioning vehicle. The FTC has taken action against a number of such products over the years.
Filling up in the morning. The theory is that cooler fuel is denser. Underground storage tanks sit at a stable temperature regardless of the time of day, so the effect is negligible.
Idling to warm up a modern engine. Fuel-injected engines reach operating temperature faster under gentle driving than at idle. Thirty seconds is plenty before driving off gently.
Air conditioning versus windows
The honest answer is that it depends on speed. At low speeds, open windows cost less than running the compressor. At highway speeds, the aerodynamic penalty of open windows generally exceeds the air conditioning load. The difference either way is modest, and comfort is a legitimate consideration.
Maintenance that affects economy
- A clogged engine air filter can reduce efficiency on older engines, though modern engine management compensates better than it used to.
- Failing oxygen sensors cause the engine to run rich, which is one of the more common causes of a sudden drop in economy.
- Dragging brakes from a sticking caliper waste fuel continuously and are often unnoticed.
- Wrong-grade engine oil increases internal friction.
A sudden unexplained drop in fuel economy is a diagnostic signal worth acting on, since it usually means something is wrong rather than something is worn.
Tested efficiency figures, fuel cost estimates and a trip calculator for most US vehicles are published at fueleconomy.gov, run jointly by the EPA and the Department of Energy.