Fuel Cost Calculator

Fuel is one of the larger running costs of a car, and unlike insurance or depreciation it is paid in small amounts often enough that most people never add it up. The arithmetic is simple: distance divided by efficiency gives the fuel used, and fuel used times price gives the cost. Everything else on this page is about the units that get in the way.

A driver covering 15,000 miles a year at 25 mpg burns 600 gallons. At $3.20 a gallon that is $1,920, and the same distance in a car doing 35 mpg costs about $1,371, a difference of roughly $550 a year for nothing but the choice of vehicle. That gap is the reason efficiency matters more than pump price for most households: you cannot control the price, and you have considerable control over the other two numbers.

Efficiency units and why they confuse people

There are two entirely different ways to express fuel efficiency, and they run in opposite directions.

Distance per unit of fuel, such as miles per gallon or kilometres per litre, gets bigger as the car gets better. Fuel per unit of distance, such as litres per 100 km, gets smaller as the car gets better. Confusing the two is the most common mistake made with this kind of calculation, and it is why this calculator asks which one you mean rather than guessing.

To convert between the two headline units, divide 235.2 by one to get the other: 30 mpg is 235.2 / 30 = 7.84 L/100 km, and the relationship works in both directions. That constant is just 100 litres per gallon per mile expressed properly, and it is worth writing on the inside of a glovebox if you buy cars across markets.

The inverse form also hides something useful, often called the MPG illusion. Going from 10 to 15 mpg saves far more fuel over the same distance than going from 30 to 35 mpg, even though both are a 5 mpg improvement. Over 10,000 miles the first saves about 333 gallons and the second about 48. If you own two cars and can only improve one, improving the thirsty one is worth roughly seven times as much. Litres per 100 km makes this obvious and miles per gallon actively hides it.

Practical ways to reduce fuel costs

Drive less

The most reliable saving is distance you do not cover. Walking and cycling use no fuel at all. Buses, trains and trams spread the fuel cost of one vehicle across many passengers, so the cost per person is far lower than everyone driving separately, and in some places local transport is free at the point of use. Once the fixed costs of owning or renting a car are counted alongside the fuel, the case for leaving it parked gets stronger still.

Carpool

Sharing a car to a common destination is the same idea at a smaller scale. A car carrying an extra passenger uses very slightly more fuel because of the added weight, and that penalty is trivial next to running a second car over the same route. Two people sharing one vehicle roughly halves the fuel cost each.

Choose a more efficient vehicle

Vehicle choice dominates everything else. Fuel cost for a small sedan is around half that of a large SUV over the same distance. The same logic applies to engines: an eight-cylinder engine costs more to run than a four-cylinder that would have done the job. Unless you regularly tow or haul heavy loads, the extra capacity is paid for continuously at the pump.

Keep the engine serviced

A car that is noticeably out of tune or has failed an emissions test typically gains about 4% in fuel economy once fixed, though the figure depends entirely on what was wrong. A serious fault is worth far more: replacing a failed oxygen sensor can improve mileage by as much as 40%.

Note that tuning for performance and tuning for economy are different jobs. Most engine tuning aims at horsepower, which is not the same target, so say which one you want.

Ornaments, aerodynamic kits and spoilers add drag without adding much else. A roof box costs 2 to 8% in town and 6 to 17% at motorway speeds, because drag rises with the square of speed. If you must carry something on the roof, angle it so the leading edge is low and the frontal area is small, and take the rack off when it is not in use.

Check the tyres

Correctly inflated tyres reduce fuel consumption by up to about 3%. Tyres lose roughly 1 PSI a month on their own, and more in cold weather as the air contracts, so a tyre that was correct in October is usually low by January. Check monthly at least, weekly if you can, and you will also get more even tread wear out of them.

Garage forecourt gauges are often inaccurate, and some automatic compressors simply stop at a preset value. Keep your own gauge and verify. Recommended pressures assume cold tyres, so add roughly 3 PSI if the car has been driven, and use the figure from the vehicle manufacturer on the door pillar rather than the maximum stamped on the tyre sidewall, which is a limit rather than a recommendation.

Use the right oil

Using the manufacturer's recommended grade of motor oil is worth 1 to 2%. Putting 10W-30 into an engine designed for 5W-30 costs meaningfully more than that, and 5W-30 in an engine designed for 5W-20 costs 1 to 2%. Look for oil marked Energy Conserving on the API symbol, which indicates friction-reducing additives.

Plan the trip

No saving beats a shorter route. Modern route planners make it easy to find the most direct option, avoid congestion and reduce stops. Motorways generally beat urban routes for economy because a steady speed suits an engine far better than repeated acceleration.

In town, park centrally once and walk between stops rather than moving the car repeatedly. Stop-start driving is the worst case for economy, and circling car parks burns fuel to no purpose. Combining several short errands into one trip also helps a great deal, because a cold engine is markedly less efficient: several short trips from cold can use twice the fuel of one longer trip covering the same distance.

Change how you drive

Driving style is the lever people underrate. Aggressive acceleration and braking costs 10 to 40% in stop-and-go traffic and 15 to 30% at motorway speeds. Fuel economy usually peaks somewhere around 50 mph and falls away above it, so each 5 mph over that mark is roughly equivalent to paying an extra 20 to 30 cents a gallon.

Idling gets you zero miles per gallon and consumes between 0.2 and 0.5 gallons an hour, so switching off beats waiting for more than a minute or so. Every 100 lb of unnecessary weight costs about 1%, which matters more on a small car than a large one. Air conditioning increases consumption noticeably in town, while at motorway speed open windows add enough drag that the air conditioning is often the cheaper option.

Cold weather is worth knowing about because it looks like a fault. Fuel economy drops around 15% at 20°F compared with 77°F, and by as much as 24% on short trips of three or four miles, since the engine spends the whole journey warming up.

Factors that determine the fuel price

Government intervention

Governments intervene in petrol markets mainly through taxation, which raises the pump price, and through subsidy, which lowers it. The size of the tax component is the single biggest reason the same barrel of oil produces very different pump prices in different countries: fuel duty and VAT make up more than half the price in much of Europe, while US federal and state taxes are a far smaller share.

Financial markets

The global oil price moves constantly. The two benchmark crudes quoted are Brent and West Texas Intermediate, priced in US dollars per barrel, and retail fuel prices follow them closely, though not symmetrically: pump prices tend to rise quickly when crude rises and fall slowly when it drops, an asymmetry economists call rockets and feathers.

Politics

Political structures, leadership and events all feed into fuel cost. A change of government can change subsidy and climate policy, and relationships between states matter too, since countries trade, form alliances and occasionally go to war over the resource. Production decisions by exporting groups move the price directly.

Geography

Some regions have abundant oil and others none. Consumers close to supply generally pay less because access is easier and transport shorter, while isolated places with no local supply, such as Pacific islands, tend to pay considerably more. Distance from a refinery affects prices within a single country too.

Natural disasters and weather

Earthquakes, hurricanes, floods and severe storms disrupt the production, refining and distribution of fuel, and any of the three moves the price. A snowstorm that closes roads drives local prices up by preventing delivery, while a hurricane that damages refineries halts production outright and pushes prices up across a much wider area.

Seasonality plays a quieter part. Many markets switch between summer and winter fuel blends, and the summer blend costs more to produce, which is one reason prices tend to firm up ahead of the driving season regardless of what crude is doing.

Using this calculator

Enter the trip distance in miles or kilometres, the efficiency in whichever of the four units your car reports, and the fuel price per gallon or per litre. The result gives the fuel needed and what it costs, along with the cost per 100 km, which is the number worth comparing between vehicles.

Underneath, the same trip is priced at a range of efficiencies. That list answers the question people actually have, which is not what this trip costs but how much a different car, or a lighter right foot, would change it.