Gas Mileage (MPG) Calculator
Your real fuel economy from one fill-up, in the unit you measured in. Plus cost per mile and what a year of driving costs.
Your fill-up
From the trip meter
At the second fill-up
This works out your real fuel economy from a tank of fuel. Enter the distance driven and the fuel it took, and it returns your actual MPG or L/100km.
It also estimates what that costs you per mile and per year, which is usually the number that changes a decision.
What gas mileage actually measures
Gas mileage is how far your vehicle travels on a given amount of fuel. In the US it is quoted as miles per gallon, where a higher number is better.
Most of the world uses the opposite: litres per 100 kilometres, where a lower number is better. Same information, inverted, which is why the two never convert by simple multiplication.
Your real figure will almost always be worse than the sticker on the window. Manufacturer numbers come from standardised test cycles, not from your commute, your right foot, or a roof box in a headwind.
Fill the tank completely, reset the trip meter, drive, then fill completely again. The fuel you put in the second time is exactly what the trip used.
What to enter
- Distance driven
- Miles or kilometres since the last fill. The trip meter is more reliable than working it out from the odometer.
- Fuel added
- How much it took to fill back up. This only works if both fills went to the same point, ideally the first click of the pump.
- Driven per year
- Used to turn the MPG figure into an annual fuel cost, which is the number worth comparing between vehicles.
Roughly what counts as good
- Under 20 MPG
- Large trucks, SUVs and older V8s. Fuel is a major running cost at this level.
- 20 to 30 MPG
- Where most petrol cars and crossovers sit.
- 30 to 40 MPG
- Efficient small cars and many diesels. Good for a conventional engine.
- 40 to 55 MPG
- Hybrids in normal use.
- L/100km
- The inverse scale. 25 MPG is about 9.4 L/100km, and 40 MPG is about 5.9. Lower is better.
What this assumes
Both fill-ups reached the same level. Topping off past the click on one fill and not the other is the biggest source of error.
One tank is a snapshot. Averaging three or four gives a figure worth trusting.
How to calculate your gas mileage
Divide distance by fuel. The measuring is what needs care, not the arithmetic.
- miles driven
- Trip meter reading since the last fill
- gallons used
- What it took to fill back to the same level
Fill up and reset. Fill to the pump's first click, then zero the trip meter.
Drive a full tank. A short trip gives a noisy result. Most of a tank gives a fair average.
Fill again and divide. Divide the trip miles by the gallons it took. That is your real MPG.
Repeat a few times. One tank of motorway driving flatters the figure. An average across several is the honest number.
See a worked example: 320 miles on 12.5 gallons, at $3.50 a gallon
- Distance
- 320 miles
- Fuel
- 12.5 gallons
- Price
- $3.50 per gallon
320 ÷ 12.5 = 25.6 MPG.
Cost per mile: $3.50 ÷ 25.6 = about 13.7 cents.
Over 12,000 miles a year that is roughly $1,641 in fuel.
In metric, 25.6 MPG is about 9.2 L/100km.
25.6 MPG, about $1,641 a year
Frequently asked questions
For a conventional petrol car, 30 MPG and up is good and 40 is very good. Hybrids commonly reach 45 to 55.
It depends heavily on the vehicle's job. 20 MPG from a large pickup is reasonable; the same from a small hatchback means something is wrong.
Official numbers come from standardised laboratory cycles, which are consistent between cars but nothing like real driving.
Short trips from cold, city traffic, high speeds, roof boxes, air conditioning, under-inflated tyres and cold weather all cut real-world economy. A 10 to 20% shortfall is completely normal.
Divide 235.2 by your MPG. So 25.6 MPG is 235.2 ÷ 25.6 = about 9.2 L/100km, and it works the same way in reverse.
You cannot convert by multiplying, because the two scales run in opposite directions: high MPG is good, low L/100km is good.
The cheapest wins are tyre pressure, removing roof racks and boxes when not in use, and clearing weight out of the boot.
Driving style matters more than any of them. Steady speeds, gentle acceleration and reading the road far enough ahead to coast rather than brake will beat most mechanical tweaks.
Only if your engine requires it. In a car designed for regular, higher-octane fuel gives no meaningful gain and simply costs more.
If the manual says premium is required rather than recommended, use it: the engine's timing is built around it, and running regular can cost both performance and economy.
Problems people actually run into
MPG is a misleading scale for comparing cars
Going from 15 to 20 MPG saves far more fuel than going from 40 to 50, even though the second improvement looks bigger. Over 12,000 miles, the first saves 200 gallons and the second only 60.
This is because MPG is upside down: it measures distance per fuel rather than fuel per distance. If you are deciding which of two cars to replace, replacing the thirsty one almost always wins, however unimpressive the MPG gain looks on paper.
Measuring one tank and treating it as gospel
A single tank driven mostly on the motorway can read 20% better than the same car in town, and neither figure describes your actual year.
Inconsistent fill-ups make it worse: stopping at the first click one time and topping off the next introduces error larger than the difference you are trying to measure.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 3, 2026