An animated tour of a free, offline fill-up log
Fuel mileage calculator, explained — how fuelbook turns fill-ups into honest tank-to-tank km/L
Everyone argues about real vs claimed mileage. fuelbook settles it with the one method that actually works for manual logging — full-to-full — and refuses to invent a number when the method can't produce one. This page shows you exactly how, with a worked example you can check by hand.
Log every full fill. The math does the rest.
The problem: nobody agrees on "real mileage"
Three numbers get shouted in every mileage argument — and all three are wrong in a different way.
The brochure figure
Measured in lab conditions. Your commute has traffic, pillion riders, and a luggage rack. Real logs routinely land well below it — but by how much, for your vehicle? Nobody's brochure knows.
The one-tank guess
"I got 55 this tank!" Every pump's auto-cutoff stops at a slightly different level, so what counts as full wobbles a few percent. Any single tank's figure is part measurement, part luck.
The spreadsheet drift
DIY sheets quietly divide by the wrong litres — counting the first fill's fuel, dropping partial top-ups, or mixing time windows. The formula looks right and the answer is confidently wrong.
The fix: the full-to-full method, animated
A full tank is the only repeatable reference point you have without instruments. So mileage is only measured between two FULL fills: kilometres travelled ÷ litres it took to refill. Watch the worked example build itself:
A segment runs between consecutive FULL fills. The litres of every fill after the previous FULL through this FULL go in the denominator — so the first FULL's 9.00 L is a starting line, never counted.
Partial top-ups fold into the segment they belong to — and the segment is honestly labelled includes partial. Segment two above: 400 km ÷ (4.50 + 5.50) L.
Entries after the last FULL are excluded, with the reason on screen. The 25 Feb partial can't be measured yet — it joins the math at your next full fill. Lifetime stays 40.83 km/L (810 km ÷ 19.84 L) either way.
No reference points, no number. One full fill, or a log of only partials? fuelbook shows your spend totals and says "need two full fills to compute" — it never invents a mileage.
One point per tank — and a trend you can act on
Single tanks wobble with pump cutoffs, so fuelbook plots every tank against a trailing-3-tank average and tells you to trust the lifetime line over any one point.
True cost per km — fuel alone understates it
fuelbook computes both figures over the same first-FULL-to-last-FULL window, so they cover exactly the same 810 km. Expenses dated outside the window are flagged waiting for next full fill, never silently mixed in.
Worked example. That gap — ₹1.49 per km here — is what bike-vs-cab-vs-metro decisions actually turn on.
Privacy you don't have to trust — the browser enforces it
Most apps promise not to send your data anywhere. fuelbook makes the promise unnecessary: its Content-Security-Policy sets connect-src 'none', so your browser itself blocks every network send. There is no server, no account, no analytics — and nothing to opt out of.
The actual policy line the app ships:
Content-Security-Policy: default-src 'self'; connect-src 'none'; …- No network path exists.
connect-src 'none'means fetch, XHR and WebSockets are refused by the browser before they leave the page. - No third-party anything. No CDN scripts, no external fonts, no analytics beacons — the policy forbids them all.
- Your log lives in
localStorage, in your browser, full stop. Clearing site data erases it; the built-in CSV export is your backup.
What's in the app
Fill-up log
Date, odometer, litres, amount — price/litre auto-derived (₹105.00/L in the example). FULL/PARTIAL toggle, edit, delete, validation that rejects backwards odometers.
Dashboard
Lifetime km/L, best and worst tank, fuel and all-in cost/km, this-month spend, total km — every card states its window.
Trend chart
One point per tank, trailing-3 average, and a neutral prompt when a tank dips more than 10% below it.
Expense log
Service, insurance, tyres, other — feeding the all-in figure over the same honest window.
Units & currency
km+L (km/L and L/100km — 41.67 km/L is 2.40 L/100km) or mi+gal (mpg). Free-text currency symbol, ₹ default.
Sample log
One tap loads clearly-invented demo entries under a persistent SAMPLE banner, so the payoff is visible before your own second full fill.
CSV export & print
RFC-4180 CSV of fills and expenses — your backup and handoff — plus a printable one-page summary.
Method card
The full-to-full rule and its limits, stated in plain words inside the app itself.
What fuelbook honestly won't do
- Manual entry only. No receipt OCR, no OBD/Bluetooth, no GPS trip tracking — and the app says so on the entry screen.
- No mileage without two FULL fills. The method can't produce one, so neither will the app.
- Single tanks wobble. Pump cutoffs vary; trust the lifetime line over any one tank.
- One vehicle per browser profile. No cloud sync, no accounts, ever — no fuel-price feeds or claimed-mileage database either. Your claimed-figure reference line is typed by you and labelled as such.
Start your log at the next full fill
Free, offline, no account. Two full fills from now, you'll have a number nobody can argue with.
Open fuelbookSource code (MIT) · runs entirely in your browser