Ranked by what it is actually worth
Typical gains, and what each is worth per year at 12,000 miles, 25 MPG and $3.29 a gallon, which comes to a $1,579 fuel bill.
| Change | Typical gain | Worth per year | Cost to do |
|---|---|---|---|
| Fix a failed oxygen sensor | up to 15% | up to $237 | $100 to $300 |
| Stop accelerating and braking hard | 10 to 25% | $158 to $395 | Free |
| Drive 65 instead of 75 mph | 8 to 14% | $126 to $221 | Free, costs time |
| Take the roof box or rack off | 10 to 20% on motorway | $100 to $250 | Free |
| Combine short trips into one | 5 to 15% | $79 to $237 | Free, needs planning |
| Replace a clogged air filter | up to 6% if actually clogged | up to $95 | $20 to $50 |
| Keep tyres at the door-sticker pressure | 1 to 3% | $16 to $47 | Free |
| Take 100 lb of junk out of the boot | about 1% | $16 | Free |
| Use the recommended oil grade | 1 to 2% | $16 to $32 | Free at the next service |
The pattern is hard to miss. The top of the table is free and behavioural, the paid items are repairs of things that broke, and none of it involves buying an accessory.
The driving half
Accelerate like the pedal costs money
This is the single biggest lever, worth 10 to 25 percent between an aggressive driver and a smooth one in the same car. Fuel goes in as acceleration and comes out as heat in the brake discs. Every hard stop you could have coasted into is fuel you bought and threw away.
Slow down on the motorway
Aerodynamic drag rises with the square of speed, so the engine has to work disproportionately harder for each extra mile per hour. Most cars are past their efficiency peak somewhere around 50 to 60 mph. Ten mph off a long drive is commonly 8 to 14 percent, and costs about eight minutes per hundred miles.
Look further ahead
Timing a red light so you arrive rolling rather than stopped uses less fuel than accelerating from a standstill. Same journey, same time, less fuel. This is most of what hypermilers are actually doing.
Combine the short trips
A cold engine can use twice the fuel of a warm one for the first couple of miles. Four separate two-mile errands are four cold starts. The same four errands on one loop is one. In winter this is one of the biggest available gains and almost nobody counts it.
The car half
Tyre pressure, checked cold, to the door sticker
Not the number on the tyre sidewall, which is the tyre's maximum rather than the car's recommendation. Check monthly and after the first cold week of the year, when every tyre on every car in the country loses a few PSI at once.
Take the rack off when you are not using it
An empty roof box costs you fuel on every single mile. Crossbars alone are worth a few percent at motorway speed. If it is not carrying something this month, it should be in the garage.
Fix the thing that broke
A failed oxygen sensor is worth up to 15 percent and often throws no warning light for a long time. A dragging brake caliper, a stuck-open thermostat and a misfire all show up as fuel economy first. This is the one category where spending money pays back, because you are not buying an improvement, you are undoing a fault.
Air filter, only if it is dirty
On a modern fuel-injected engine, a genuinely clogged filter is worth a few percent. A filter that merely looks a bit grey is worth nothing. Replacing it on schedule is fine maintenance and a poor mileage strategy.
Things that do nothing
| Claim | Reality |
|---|---|
| Premium fuel in a regular-fuel car | Octane is knock resistance, not energy. No gain, higher price. |
| Fuel-line magnets, vortex generators, "chips" | Tested extensively for decades. Nothing works. |
| Mileage-boosting additives | No independently verified gains. Detergent cleaners are maintenance, not mileage. |
| Filling up in the morning when fuel is denser | Underground tanks barely change temperature. The effect is far too small to measure. |
| Over-inflating the tyres | A fraction of a percent, in exchange for worse grip, worse braking and uneven wear. |
| Coasting in neutral downhill | Modern engines cut fuel entirely on a closed throttle in gear. Neutral makes it worse, and it is illegal in some places. |
| Warming the car up for five minutes | Necessary on a carburettor in 1985. A modern engine warms faster under load and idling just burns fuel. |
If you want to test any of these yourself, the method is the same as measuring anything else: five tanks before, five tanks after, same season, same commute. Almost every claimed gain disappears once it is up against that.