Calculator of Alexandria

Fuel cost calculator

Work out the fuel cost of a road trip from distance, fuel economy and price, in mpg or L/100 km, split between passengers.

Units
Trip fuel cost
Fuel used (gallons or litres)
Cost per person
Economy in the other units (L/100 km or mpg)

Formula

cost=dmpg×p\text{cost} = \frac{d}{\text{mpg}} \times p
How it's derived

The formula

cost=dmpg×p\text{cost} = \frac{d}{\text{mpg}} \times p cost=d×L/100 km100×p\text{cost} = \frac{d \times \text{L/100 km}}{100} \times p

dd is the trip distance and pp the price per gallon or per litre.

Where it comes from

Two steps: work out how much fuel the trip needs, then price it.

Miles per gallon says how far one gallon goes, so the gallons needed are distance divided by mpg. L/100 km works the other way round. It says how much fuel each 100 km uses, so the litres needed are the distance in hundreds of km times that figure.

The two measures are reciprocals. Converting between them uses the litres in a US gallon (3.785) and the km in a mile (1.609):

L/100 km=100×3.7851.609×mpg=235.2mpg\text{L/100 km} = \frac{100 \times 3.785}{1.609 \times \text{mpg}} = \frac{235.2}{\text{mpg}}

Worked example

A 350-mile trip in a car that gets 28 mpg, with gas at $3.45:

35028=12.5 gallons\frac{350}{28} = 12.5\ \text{gallons} 12.5×3.45=$43.1312.5 \times 3.45 = \$43.13

Split between three people, that’s $14.38 each.

Watch out for

  • Real-world economy. Highway driving, heavy loads, roof boxes and cold weather all change it. Your car’s trip computer is a better guide than the sticker figure.
  • Improvements aren’t linear in mpg. Going from 10 to 20 mpg saves far more fuel than going from 40 to 50, which is why many countries use L/100 km.

Sources

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