Gas Mileage & Fuel Cost Calculator
Enter one fill-up and this gives you MPG for that tank, what the fuel actually costs per mile, and what a year of driving comes to. Log each fill-up and it starts answering from your own numbers instead of the window sticker. Everything stays on this device: no account, no login, nothing sent anywhere.
Your fill-up log
Fuel economy swings with weather, tire pressure, traffic and how full the tank was, so a single tank means little. Three or four logged fill-ups in a row is where the average starts telling the truth.
What this actually tells you
The trip computer on the dash gives you a number, and on many cars that number is optimistic by a few percent because it is calculated from injector pulses rather than from fuel you paid for. Odometer miles divided by gallons on the receipt has no such bias. It is the same method fleet managers have used for decades, and it is the only one your wallet recognizes.
The second thing this page gives you is cost per mile for fuel alone, which is the number that makes a longer commute or a cheaper station easy to judge. Ten cents a mile against thirteen sounds small until you multiply it by 12,000 miles a year.
What a falling average is telling you
A tank that comes in five or ten percent under your own average is usually the weather or a run of short trips. A gradual slide over several tanks is worth investigating: under-inflated tires, a clogged air filter, dragging brakes and a tired oxygen sensor all show up here before they show up as a warning light. That is the real argument for logging every fill-up — the chart notices the slide before you do.
Questions people ask
How do I calculate MPG by hand?
Fill the tank, note the odometer, drive, then fill it again to the same point. Divide the miles you covered by the gallons it took to refill. That second fill-up is the key part — the gallons you just put in are exactly what the previous stretch of driving used.
Why is my MPG lower than the EPA rating?
EPA numbers come from a standardized lab cycle. Cold starts, short trips, roof racks, winter fuel blends, low tire pressure and a heavy right foot all cost real miles per gallon. Being ten to fifteen percent under the sticker is completely ordinary.
Do partial fill-ups ruin the calculation?
They skew any single tank, because you no longer know exactly how much fuel the last stretch used. Filling to the same click every time keeps the math honest. If you do take a partial fill, log it anyway — the running average absorbs it over a few tanks.
What is a good fuel cost per mile?
It depends entirely on your car and local fuel prices. A car doing 30 MPG with fuel at $3.29 a gallon costs about 11 cents a mile in fuel. The useful comparison is not someone else’s number but your own, a few months apart.
Is my fill-up log stored anywhere?
No. It lives in your browser’s local storage on this device only. Nothing is uploaded, and you can download or print the log any time.