You filled the tank on Tuesday. Same pump, same amount, same route to work. The receipt read ₹120 more than last month, and the price-tracker app on your phone still says petrol is down two rupees a litre from where it was. You blame the traffic, or the season, and fill up again next week. What actually happened is your car started working harder for the same kilometre sometime in the last two or three months — and there was no receipt for that.
Why nobody notices
A fuel bill only tells you what you spent, not what you got for it. Two tanks a month apart can cost the same amount and still mean a real drop in how far you went on them — because nobody remembers the odometer reading from the last fill, and nothing is doing that subtraction. The number that would catch this, cost per kilometre, takes an odometer photo and a calculator most people never open.
The usual suspects
Tyre pressure under the recommended number pushes the engine to work harder for the same distance, and it's the cheapest thing on this list to check — two minutes at any pump, against the number on the sticker inside the driver's door, not the tyre's sidewall. A clogged air filter starves the engine of air and it compensates by burning more fuel; most manufacturers call for a look every 10,000–15,000 km. Worn spark plugs or an overdue service is the one that costs money to fix, and the one that gets caught last, because it looks like normal wear rather than a sudden problem.
What would actually catch it
The fix isn't remembering to check more often — it's having something else do the noticing. One ₹120 jump doesn't mean anything on its own. Three fills in a row costing more per kilometre than your own last month does. That comparison only works against your own car's history, not a city average, because two cars of the same model driven differently have different honest baselines.
Do this before the next fill
Check tyre pressure cold, against the driver's-door sticker. If the air filter hasn't been looked at in the last 10,000 km, ask for it at the next service. And log the next few fills somewhere that remembers the last one, so the next jump gets caught in week one instead of month three.