About the MPG Calculator
The miles per gallon printed in a car's brochure is measured in a laboratory, on a rolling road, following a fixed test cycle. It is useful for comparing cars, but it is rarely what you actually get. Your real fuel economy depends on your roads, your traffic, your climate and the way you drive. The only way to know it is to measure it, and the method is simple enough to do at any fuel station.
This MPG calculator uses the fill-up method. Enter the odometer readings at two fill-ups, or the miles from your trip meter, and the fuel you added to refill the tank. It works out your real miles per gallon, converts it to litres per 100 km and km per litre, shows the figure in the other kind of gallon, and — if you add the fuel price — your fuel cost per mile. It works with US gallons, or with litres for UK mpg.
How to Use the MPG Calculator
Choose whether to enter odometer readings or miles driven.
Enter the odometer at the first and second fill-ups, or the miles from the trip meter.
Choose whether the fuel is measured in US gallons or litres (for UK mpg), and enter the fuel added at the second fill-up.
Optionally, enter the fuel price to see the cost per mile.
The Fill-Up Method
- Fill the tank until the pump clicks off, and note the odometer or reset
the trip meter.
- Drive normally until you need fuel again — ideally most of a tank.
- Fill the tank again, the same way, and note the fuel added and the new
odometer reading.
The fuel added at the second fill-up is exactly the fuel the car used to drive those miles, so dividing one by the other gives your real mpg.
How MPG Is Worked Out
miles = second odometer reading − first reading
mpg = miles ÷ gallons added
UK gallons = litres ÷ 4.546
L/100 km = 235.215 ÷ US mpg (282.481 ÷ UK mpg)
cost/mile = cost of the fill-up ÷ milesStep-by-Step Example
Odometer 12,000 then 12,350, with 11.2 US gallons added at 3.40 a gallon.
Miles: 12,350 − 12,000 = 350
MPG: 350 ÷ 11.2 = 31.25, about 31.3 mpg
L/100 km: 235.215 ÷ 31.25 = 7.53
Fill-up: 11.2 × 3.40 = 38.08
Per mile: 38.08 ÷ 350 = 10.9 cents400 miles on 40 litres in the UK.
Gallons: 40 ÷ 4.546 = 8.8 UK gallons
MPG: 400 ÷ 8.8 = 45.5 mpg
L/100 km: 282.481 ÷ 45.5 = 6.21Getting an Accurate Figure
Fill the same way each time. Stop at the first click of the pump rather than topping up, and use the same pump if you can. Topping up to different levels is the biggest source of error.
Measure over a full tank. The longer the distance between fill-ups, the smaller the effect of any small difference in how full the tank is.
Average several fill-ups. Add up the miles and the fuel over three or more tanks and divide the totals, rather than averaging the separate mpg figures.
Check the odometer. Odometers can read a few percent high or low, especially with non-standard wheel sizes. A GPS app over a known distance will reveal this.
Why Your MPG Differs From the Official Figure
Official tests are carried out indoors at mild temperatures, with no air conditioning, no passengers and gentle, standardised acceleration. On the road, cold mornings, short trips before the engine warms up, heavy traffic, hills, roof racks, under-inflated tyres, heavy loads and motorway speeds all increase fuel use. A real figure 10 to 20 percent below the official one is normal. A sudden drop in your own measured mpg, on the other hand, can be an early sign of a problem such as a dragging brake, a failing sensor or low tyre pressure.
Improving Your MPG
Keep tyres at the correct pressure, remove roof racks and unnecessary weight, accelerate gently, anticipate the traffic so you brake less, and keep to a moderate cruising speed. Combining short errands into one trip helps, because a warm engine is far more efficient than a cold one. Regular servicing keeps the engine running efficiently. Together these habits can improve real-world economy by 10 percent or more.
Keeping a Fuel Log
A simple fuel log — date, odometer, fuel added and price — turns single readings into a clear picture of how your car performs through the seasons. Many drivers see economy fall by 10 percent or more in winter and recover in spring. The log also gives a reliable record of business mileage and running costs, and it is useful evidence of careful maintenance when you come to sell the car.
Understanding Your Result
Miles per gallon is your real fuel economy for this tank.
Distance and fuel confirms the miles driven and the fuel added.
L/100 km gives the figure in litres per 100 km and km per litre.
Other gallon gives the same economy in the other kind of gallon.
Cost shows the cost of the fill-up and the cost per mile.
Worth knowing explains why you should average several fill-ups.
When Should You Use This Calculator?
Checking a new car against its official figure.
Tracking economy over months to spot problems early.
Working out the cost per mile for business mileage.
Comparing driving habits, routes or tyres.
Converting your mpg to litres per 100 km.
Common Mistakes
Topping up to different levels at each fill-up.
Measuring over a short distance.
Averaging mpg figures instead of totals.
Mixing US and UK gallons.
Trusting the dashboard figure without checking it.