Fuel Economy & Fuel Cost Calculator

What fuel economy means

Fuel economy is just a way of saying how far your vehicle goes on a certain amount of fuel. If you ride a motorbike to work and fill up once a week, you already care about it, even if you've never given it a name.

There are two ways to say it. Some countries count distance per fuel: kilometres per litre (km/L) in Japan, India and Brazil, miles per gallon (MPG) in the US and UK. Others count fuel per distance, like litres per 100 kilometres (L/100 km) in Europe and Australia. Same idea, flipped upside down. With km/L and MPG, a bigger number is better. With L/100 km, a smaller number is better.

How to calculate your mileage properly

The sum itself is easy: distance ÷ fuel used = mileage. The trick is getting honest numbers for the two parts, and the best way is the full-tank method.

Fill the tank until the pump clicks off and write down your trip meter (or reset it to zero). Drive normally for a few days. When you fill up again, stop at the same click-off point, note the litres the pump says you added, and read the trip meter. That distance divided by those litres is your real mileage between two fills.

One fill-up can mislead you, because nozzles click off at slightly different points. Do it three times and average the results. That's the figure to trust.

A worked example

Why L/100 km and MPG feel opposite, and how to convert

They are reciprocals of each other. Fuel per distance goes down as a vehicle gets thriftier. Distance per fuel goes up. A car doing 20 km/L uses 5 litres per 100 km. A car doing 10 km/L uses 10. Doubling km/L halves L/100 km.

To switch between them, divide 100 by the number you have: L/100 km = 100 ÷ km/L and km/L = 100 ÷ L/100 km. Here are a few common values, worked out with the exact constants.

Distances and speeds come up in the same way. Our length converter, volume converter and speed converter cover the rest.

US gallons vs UK gallons

Here's where people get caught out. A US gallon is 3.785 litres. A UK (imperial) gallon is 4.546 litres, about 20% bigger. Since MPG is miles per gallon, the same vehicle always shows a higher MPG in the UK system. Our 14.29 km/L example is 33.6 MPG in US gallons but 40.4 in UK gallons.

So when you read "45 MPG" in a review, check which gallon it means. A quick rule: UK MPG ÷ 1.2 = US MPG. If you'd rather skip the arithmetic, switch the gallon option at the top of the calculator.

How to work out your trip cost and cost per km

Trip cost needs three things: the distance, your mileage and the price of a litre. Fuel needed = distance ÷ mileage. Cost = fuel needed × price. Cost per km is simply price ÷ mileage, and it doesn't depend on the trip at all.

Thinking about a loan for a newer, thriftier vehicle? Fuel savings are only one side of it, so run the repayments through our loan EMI calculator too. And if you want to see what a percentage saving means in money, the percentage calculator does it in a click.

Real-life things that change your mileage

If your numbers bounce around from one fill-up to the next, that's normal. Here's what moves them.

Tyre pressure. Soft tyres drag. Check them every couple of weeks, cold, and use the pressure on the sticker inside the door or in the manual. Speed. Air resistance climbs fast, so a steady 60 to 80 km/h usually beats 100 and above. Traffic. Stop-start crawling in Pettah at rush hour burns far more than an open road, because the engine works hard to move a stationary weight again and again.

Air conditioning adds load, especially in slow traffic. Weight matters too: that extra bag of cement or a roof rack you never use costs you a little on every trip. Servicing counts, since clogged air filters, old spark plugs and dirty injectors all waste fuel. Then there's driving style. Hard braking and flat-out acceleration can cost you a lot over a month. Even fuel quality and the weather play a small part.

Petrol, diesel, hybrid, LPG: how the numbers compare

Be careful here, because the fuels aren't measured the same way. A litre of diesel holds more energy than a litre of petrol, so diesel vehicles often show a better km/L. That doesn't automatically make them cheaper to run. It depends on the price of each fuel where you live, and diesel vehicles can cost more to buy and service.

Hybrids use an electric motor to help the petrol engine, which saves the most in stop-start traffic where a normal engine wastes a lot. On a long, steady highway run the gap narrows. LPG and CNG vehicles often show a lower distance per litre or kilo, but the gas is usually cheaper, so cost per km is the fairer measure.

When you compare two fuels, always compare cost per km, not the mileage figure alone.

A small habit makes this easier. Whenever you fill up, write down three things: the odometer reading, the litres and the price per litre. Keep them in a notes app or on a scrap of paper in the glovebox. After five or six fill-ups you'll have a real picture of your vehicle, and one odd tank won't fool you. Someone who rides a small motorbike to work and someone who drives a small car on the school run are doing very different sums, and both are fine. What matters is that the numbers come from your own pump receipts, not from a brochure.

Motorbike, three-wheeler, car, van: what's "normal"?

Treat these as very rough ranges, not promises. Condition, load and roads matter more than the label on the vehicle.

If you're below your range, look at tyres, servicing and how you drive before you blame the engine. If you're above it, well done.

10 practical ways to improve your mileage and cut your fuel bill

• Check tyre pressureEvery two weeks, when the tyres are cold.
• Drive smoothlyEase onto the pedal and brake early.
• Hold a steady speedAvoid racing between traffic lights.
• Service on timeClean filters and good plugs matter.
• Lose the extra weightEmpty the boot and remove the unused rack.
• Use A/C sensiblyWindows down at low speed, A/C on at highway speed.
• Don't idleSwitch off if you'll be stopped for a minute or more.
• Combine errandsOne longer trip beats several cold starts.
• Plan your routeSkip the worst traffic and the steepest hills.
• Track every fill-upYou can't improve what you never measure.

Try the budget tab's slider to see what a 10% gain is worth to you. For many people it's a noticeable amount each month, which is worth a few minutes with a tyre gauge.

How reliable are claimed mileage figures?

Claimed figures usually come from standardised tests in controlled conditions. They're useful for comparing one vehicle with another, but they rarely match what you'll see on a hot day in heavy traffic with the A/C running and four passengers aboard. A shortfall of 10 to 25% against the claim isn't unusual, though it varies. Your own full-tank figure is the one that reflects your roads and your habits, so trust it over a brochure, and use the brochure as a starting point only.

If you want to check a claim yourself, do it the slow, honest way. Fill the tank until the pump clicks off the first time, reset your trip meter, and drive as you normally do. Fill up again at the same pump, stopping at the first click, and divide the distance by the litres. Do it three times and take the middle result. It takes a few weeks, but it tells you more than any leaflet, because it includes your roads, your traffic and your habits.

Common mistakes when calculating mileage

• Judging by one fill-up instead of averaging three or more.
• Not filling to the same point each time, which shifts the litres by a few percent.
• Trusting the dashboard blindly. Computers can read optimistic.
• Mixing units, for example miles with litres or US gallons with UK ones.
• Comparing mileage across fuels without looking at cost per km.
• Forgetting short trips. Cold starts burn more fuel per kilometre.
• Using an old fuel price in your cost sums. Always use today's.

Frequently asked questions

Disclaimer, method and sources

Results here are estimates for planning. Real mileage varies with the vehicle, load, road, weather and driving style, and fuel prices change often, so we never fill a price in for you. This page doesn't give professional advice.