Fuel log vs memory: what actually tracking your mileage reveals

Ask a driver what their car's fuel economy is and you get a number. Check it against a fuel log and the number is usually 10 to 20 percent too optimistic. Ask what they spend on fuel each month and the gap is bigger. Memory is a poor instrument for anything that happens weekly, varies by trip and never produces a bill. A fuel log, kept for three months, replaces the story with the figures, and the figures are rarely what people expect. Here is what shows up.

1. Your real fuel economy, which is lower

The brochure figure was measured in a lab. The dashboard figure is an estimate from injector timing that tends to flatter. Your remembered figure is the best tank you ever had. The log gives you the average across every kind of trip you actually make, including the cold starts and the school run, which is the only figure that predicts next month's spend. For most city drivers it lands 15 to 25 percent under the brochure.

2. Your real fuel spend, which is higher

Fuel is bought in small amounts, often, with a card, and never arrives as a monthly bill. People who guess ₹5,000 a month find ₹7,500 in the log. Once the number is real, two things happen: it goes into the household budget properly, and the question "is the car worth it for this trip?" starts getting asked. What a car really costs is usually the next thing people look up.

3. The city-highway split

One tank at 11 km/L and the next at 19 km/L looks like noise until you see that the first was a fortnight of 4 km commutes and the second was one trip to the coast. The log shows how much of your driving is the expensive kind, and whether a scooter, a cab or walking would cover the worst of it.

4. The season

Winter fuel economy is 5 to 15 percent worse: cold engines run rich, air is denser, heaters and demisters draw power, tyres lose pressure. Summer brings air-conditioning. A log over a year shows the curve, which stops you worrying about a dip that is simply December.

5. When something is wrong with the car

This is the one that pays for the habit. Fuel economy drifting down over several tanks, with no change in your driving, is the earliest and cheapest warning you get of a problem. In rough order of likelihood: tyre pressure down a few psi; a clogged air filter; spark plugs past their life; a brake caliper dragging; wheel alignment knocked out by a pothole; an oxygen sensor failing. All of these cost little to fix early and a lot to ignore. Without a log you notice at the next service, if the garage mentions it.

6. Whether the last service helped, or hurt

Fuel economy should improve a little after a service with new plugs and filters. If it falls, something was not done or was done badly. If it falls sharply, the garage may have left something dragging. The log turns a vague "it feels a bit sluggish" into a number you can take back to them.

What good logging looks like

• Fill to the first click, every time, so "full" is consistent.
• Log the odometer, the litres and the price at every fill, including partial ones. Economy is calculated between full fills; cost needs every fill.
• Do it at the pump. A fill logged tonight is a fill logged wrongly or never.
• Look at the trend over three or four fills, not at a single tank.
• When the number moves, ask why before you ask the garage.

The honest comparison

Memory is free and effortless and wrong by a fifth. A log costs twenty seconds a week and tells you what the car costs, how it is being driven, and when it needs attention before it becomes expensive. There is no version of car ownership where that trade is a bad one.