Mileage isn't a number. It's a signal.
"My car gives 14." It gives 14 this month. Mileage moves with tyres, traffic, season, and the car's health — and the trend over time is the diagnostic. Here's how to read it, and why your fuel price history belongs on the same chart.
What a falling trend is trying to tell you
One low reading means nothing — traffic, a heavy week, a partial fill. A trend means something. The usual suspects, roughly in order:
- Tyre pressure. The cheapest fix and the most common cause. Even a few PSI down measurably drags mileage — check before suspecting anything mechanical.
- Driving mix and season. More short cold-start trips, winter fuel blends, AC season — cold-weather, short-trip driving can knock a substantial chunk off, and it's not a fault.
- Air filter. Clogged filters choke airflow; mattered more in the carburetor era, still noticeable under load.
- Spark plugs and oxygen sensor. Aging ignition and a lazy O₂ sensor richen the mixture quietly. If plugs are past their interval, this is the classic slow decline.
- Measurement noise. Partial fills, different pumps, tank not truly full. The trend chart averages this away — which is the point.
The diagnostic power comes from correlation: mileage stepped down right after a tyre change? Brand or pressure. Gradual decline across months? Filters, plugs, sensor. Dips every winter and recovers? Season, harmless. You can only see these patterns on a chart of your own history — which is exactly what a tracker is for.
The other half: fuel price history
Your cost per km is two variables multiplied: how much fuel the car used, and what it cost that day. When your monthly fuel bill jumps, which one moved?
- Price went up, mileage flat → nothing wrong with the car; maybe shift filling days or stations.
- Mileage fell, price flat → the car wants attention (see above).
- Both moved → the chart keeps you from blaming the car for the pump's sins.
Over a year, your own price-per-litre history is also just interesting: you see the spikes and the station differences you actually experienced, not the national average headline.
How long before the trend means something?
Three or four full-tank fills give a usable line; two to three months of regular logging smooths the noise. Judge trends, not points — and log every fill so the chart has material to work with.
Common questions
Why did my mileage suddenly drop?
In order of likelihood: tyre pressure, driving mix (more city/cold/heavy), air filter, plugs or oxygen sensor, or just noise from a partial fill. A trend chart narrows it down; a single reading can't.
Does mileage drop in winter?
Yes, and it's normal — cold starts, winter fuel blends and idling all take a cut. If it recovers in spring, nothing is wrong.
How do I track fuel price changes over time?
Log price per litre (or per gallon) at every fill — FuelCap records it automatically with each entry, and charts your effective cost per km alongside the mileage trend. Free on every plan.