Fuel Calculator

How much fuel does a trip need?

To budget a drive, you need two figures: how much fuel the car will burn, and what that fuel costs. Both follow from three inputs: the distance, the vehicle's fuel consumption and the price per unit of fuel (the price is optional).

Pick the unit system first. With US units the distance is in miles, fuel is in gallons and the consumption is given in miles per gallon (mpg) or gallons per 100 miles. With metric units the distance is in kilometres, fuel is in litres and the consumption is in kilometres per litre or litres per 100 km. Choosing a unit system also sets the consumption unit to miles per gallon (US) or kilometres per litre (metric); change it afterwards if your figure is in gallons per 100 miles or litres per 100 km. The consumption unit has to belong to the same system as the distance; the calculator tells you if the two do not match.

Example 1 (US units). A 300 mile trip in a car that returns 30 mpg needs 300 ÷ 30 = 10 gallons. At $3.50 per gallon the fuel costs 10 × 3.50 = $35.

Example 2 (metric units). A 450 km trip at 7.5 litres per 100 km needs 450 ÷ 100 × 7.5 = 33.75 litres. With fuel at 1.65 per litre, the cost is 33.75 × 1.65 = 55.69.

Formula for the fuel amount

There are two families of consumption figures, and each has its own formula. In the first, a bigger number means better economy (distance per unit of fuel). In the second, a bigger number means more fuel burned (fuel per fixed distance).

fuel = distance ÷ economyFor miles per gallon (mpg) and kilometres per litre (km/l).

fuel = distance ÷ 100 × consumptionFor gallons per 100 miles and litres per 100 km.

cost = fuel × price per unit

The cost is worked out from the exact fuel amount, not from the rounded figure shown. Fuel is displayed to two decimals, and whole numbers are shown without decimals.

If your car's figure is in a different style, convert it first. As a rule of thumb, 235.2 ÷ mpg(US) gives litres per 100 km, and 100 ÷ km/l gives litres per 100 km as well. For example, 30 mpg is about 7.84 L/100 km, and 15 km/l is 6.67 L/100 km. The fuel consumption converter does this for any value.

Why real fuel use differs from the estimate

The calculator multiplies one average figure by the distance, but fuel use changes from trip to trip. The main reasons:

• Speed. Air drag grows with the square of speed, so cruising at 120 km/h instead of 100 km/h means about 44% more drag.
• Driving style and traffic. Hard acceleration and stop-and-go traffic use much more fuel than steady cruising.
• Load and equipment. Passengers, luggage, roof boxes and towing raise consumption.
• Conditions. Cold weather, headwinds, hills, low tyre pressure and air conditioning all cost fuel.

A practical habit is to add a safety margin of 10 to 15 per cent. On the 450 km example above, 33.75 litres plus 15% is 38.81 litres, which tells you whether one tank is enough or whether to plan a stop.

To find your own real-world consumption, fill the tank, reset the trip meter, drive normally and use the fuel consumption calculator or the MPG calculator the next time you fill up.

Tips for improving fuel economy

• Keep tyres inflated to the pressure on the door placard. Under-inflated tyres increase rolling resistance.
• Drive smoothly: anticipate traffic, accelerate gently and coast to slow down instead of braking late.
• Keep to moderate highway speeds. The saving from dropping 10 km/h on a long motorway leg is noticeable.
• Remove roof racks and heavy items you do not need. Extra weight and extra drag both cost fuel.
• Avoid long idling, and combine short errands into one trip so the engine is warm.
• Service the vehicle on schedule. A clogged air filter, old spark plugs or wrong-grade oil can all reduce economy.

Combine the result with the average speed calculator to estimate the driving time for the same trip.