Fuel Log App: Track MPG and Fuel Costs

Updated 2026-07-28

One fill-up tells you almost nothing. MPG on a single tank swings with traffic, weather, a low tire, or a heavy trunk — read too much into it and you'll chase noise instead of a real trend. What matters is what happens over ten or twenty fill-ups, once the random swings average out and a pattern in mileage or cost per gallon comes into view. A fuel log is what turns that pattern into something you can see, instead of a drawer of receipts or a vague memory that "gas felt expensive last month."

What a fuel log should capture

A useful fuel log needs a handful of numbers at every fill-up: the date, the odometer reading, the amount of fuel added in liters or gallons, and the total price paid. The odometer reading is the one people forget — the receipt already has the volume and the price printed on it, but the mileage only shows up if you note it yourself before pulling away from the pump. With odometer readings from two consecutive full tanks, you can work out the distance driven between them; divide that by the fuel added on the second fill-up and you have miles per gallon, or liters per 100 km, for that stretch of driving. Partial fill-ups complicate the math. If you stop and add half a tank rather than filling to the top, that entry can't be used to calculate an accurate mileage figure on its own, because the calculation assumes the tank started and ended at the same reference point — full. Some drivers skip logging partial fill-ups for the mileage calculation and only use them to track spending; others always fill to the top so every entry is usable for both. Either approach works, as long as it stays consistent.

At minimum, record these at every fill-up:

• Date of the fill-up
• Odometer reading at the pump
• Volume added, in liters or gallons
• Total price paid, or price per unit
• Whether the tank was filled completely or topped off partway

Why one tank won't tell you much

MPG on any single tank moves for reasons that have nothing to do with how well the car is running: a week of stop-and-go city driving instead of highway trips, cold weather, underinflated tires, a roof box or a trunk full of gear, or running the air conditioner hard on a summer commute. Any one of these can shift a single fill-up's number by a wide margin, in either direction. Compare last week's fill-up to the one before it and you might conclude the car got suddenly worse, or suddenly better, when really the driving conditions changed and not the car itself. The value of a fuel log shows up after ten or twenty entries, once those swings average out and a real trend becomes visible. A mileage number that drifts down gradually across many consecutive fill-ups, rather than during one rough week, is worth a closer look — it can point to something mechanical, like underinflated or worn tires, a clogged air filter, or a maintenance item that's overdue, rather than to how or where you were driving. The same log answers a second question that's easy to lose track of: how much fuel costs across a year of ownership. Paid a tank at a time, fuel spending disappears into a dozen small transactions that never add up to a single number in your head; logged and totaled, it becomes a monthly or yearly figure you can compare against insurance, service, and other running costs. CarCare's fuel log, part of the Family plan, is built around this same fill-up-by-fill-up approach: enter the date, odometer, volume, and cost each time you fill up, and CarCare works out MPG for that fill-up and tracks fuel cost over time, so the trend is visible without exporting anything to a spreadsheet. Because CarCare supports multiple cars, each vehicle keeps its own fuel log, which matters if you're comparing a commuter car against one used mostly for highway trips — the two will show genuinely different mileage patterns, and combining them would hide both. Fuel numbers mean more next to the rest of what a car costs to run: CarCare's expense tracking, also on the Family plan, puts fuel alongside service, insurance, and tire costs, with cost-per-km analytics that roll it all into one total cost of ownership figure instead of several separate ones.

A few patterns in the numbers are worth a second look:

• Mileage that trends down over several consecutive fill-ups, not only one
• A fill-up noticeably larger than usual with no change in driving habits
• Cost per fill-up climbing well beyond local fuel price changes
• A shift in mileage right after a repair or tire change, showing whether the work had the intended effect

CarCare does not provide legal, insurance, traffic, or mechanical advice. Always confirm deadlines and requirements with your owner's manual, a qualified mechanic, or your local authority.