Skip to main content
Kalcufy

Fuel Cost Calculator

Work out what a trip costs in fuel — in US mpg, imperial mpg, L/100 km or km/L — and see why going from 15 to 20 mpg saves more than 40 to 50.

Reference table

DistanceFuel for this tripFuel usedCost per mile / km
1$0.250.07 gal · 0.3 L$0.123 per mile
2$0.490.14 gal · 0.5 L$0.123 per mile
3$0.740.21 gal · 0.8 L$0.123 per mile
4$0.990.29 gal · 1.1 L$0.123 per mile
5$1.230.36 gal · 1.4 L$0.123 per mile
10$2.460.71 gal · 2.7 L$0.123 per mile
15$3.701.07 gal · 4.1 L$0.123 per mile
20$4.931.43 gal · 5.4 L$0.123 per mile
25$6.161.79 gal · 6.8 L$0.123 per mile
30$7.392.14 gal · 8.1 L$0.123 per mile
40$9.862.86 gal · 10.8 L$0.123 per mile
50$12.323.57 gal · 13.5 L$0.123 per mile
60$14.794.29 gal · 16.2 L$0.123 per mile
70$17.255 gal · 18.9 L$0.123 per mile
75$18.485.36 gal · 20.3 L$0.123 per mile

⚠️ The MPG Illusion, in Numbers

  • 15 → 20 mpgsaves 1.67 gal / 100 mi
  • 20 → 25 mpgsaves 1.00 gal / 100 mi
  • 25 → 30 mpgsaves 0.67 gal / 100 mi
  • 30 → 40 mpgsaves 0.83 gal / 100 mi
  • 40 → 50 mpgsaves 0.50 gal / 100 mi
  • VerdictThe +5 at the bottom beats the +10 at the top

What Actually Moves the Number

  • MPG is an inverted ratio, so equal steps do not save equal fuel. Replacing a 15 mpg vehicle with a 20 mpg one saves more petrol over 100 miles than replacing a 40 with a 50.
  • Litres per 100 km is the honest form: it is consumption over distance, so the difference between 10 and 9 is the same saving as between 6 and 5.
  • An imperial gallon is 20% larger than a US one, so 45 mpg in Britain is only about 37 mpg in America. Comparing the two figures directly is always wrong.
  • Manufacturer figures are lab numbers. Real-world consumption typically runs 10–20% worse, and worse still in winter, in traffic, or with a roof box.

Two Ways to Say the Same Thing, Backwards

Fuel economy can be written as distance per fuel — miles per gallon, kilometres per litre — or as fuel per distance, which is what litres per 100 kilometres means. They describe the same physical fact, but they run in opposite directions: with miles per gallon, higher is better, and with litres per 100 kilometres, lower is better. That inversion is not a formatting quirk. It changes which differences look big, and it is the reason two drivers can look at the same pair of cars and disagree about which upgrade saves more, without either of them making an arithmetic mistake.

Why 15 to 20 Beats 40 to 50

Over 100 miles, a vehicle at 15 mpg burns 6.67 gallons and one at 20 mpg burns 5.00 — a saving of 1.67 gallons. A vehicle at 40 mpg burns 2.50 gallons and one at 50 burns 2.00, saving just 0.50. The 5 mpg improvement at the bottom of the range saves more than three times as much fuel as the 10 mpg improvement at the top, because you are dividing rather than multiplying. Larrick and Soll named this the MPG illusion in Science in 2008, and it is why the US Environmental Protection Agency now prints gallons per 100 miles alongside mpg on the window sticker of every new car. Europe never had the problem, because litres per 100 kilometres was already the right way round.

What Makes Real Consumption Worse Than the Sticker

  • Speed: consumption climbs steeply above roughly 100 km/h. Dropping from 130 to 110 km/h can cut fuel use by 15–20%.
  • Cold starts: the first few kilometres of a cold engine can use double the fuel. Short trips are the most expensive per kilometre you will ever drive.
  • Tyre pressure: under-inflated tyres add rolling resistance and cost a few percent, checkable in two minutes.
  • A roof box or bike rack adds serious drag — commonly 10–25% at motorway speed. Take it off when you are not using it.
  • Air conditioning costs a few percent; an open window at motorway speed can cost more than the air conditioning would.
  • Weight matters less than people think for a car, and a great deal for stop-start city driving with a full load.

The Four Conventions, and Which Countries Use Them

  • US mpg: miles per US gallon of 3.785 litres. Used in the United States.
  • Imperial mpg: miles per imperial gallon of 4.546 litres. Used in the UK, so British figures look about 20% better than American ones for the same car.
  • Litres per 100 km: the standard across the EU, and the only one where a smaller number is better.
  • Kilometres per litre: used in Japan, India, Brazil and much of Latin America. Same direction as mpg, metric units.
  • Canada shows both L/100 km and imperial mpg on the same label, which catches out cross-border shoppers regularly.
  • You cannot average two mpg figures by adding and halving. Convert to fuel-per-distance, average that, then convert back.

Worked Examples

A commute and the illusion

A 30-mile commute, there and back

  1. 30 × 2 = 60 miles at 28 mpg
  2. 60 ÷ 28 = 2.14 US gallons
  3. 2.14 × $3.45 = $7.39

$7.39 a day, about $160 a month

Which upgrade saves more over 100 miles?

  1. 15 mpg → 6.67 gal; 20 mpg → 5.00 gal. Saved: 1.67 gal
  2. 40 mpg → 2.50 gal; 50 mpg → 2.00 gal. Saved: 0.50 gal
  3. The +5 mpg beats the +10 mpg, three times over

Fix the thirstiest vehicle first

Frequently Asked Questions

How do I calculate the fuel cost of a trip?

Divide the distance by the fuel economy to get the fuel used, then multiply by the price. 60 miles at 28 mpg is 2.14 gallons, and at $3.45 a gallon that is $7.39.

What is the MPG illusion?

Because miles per gallon is an inverted ratio, equal improvements do not save equal fuel. Going from 15 to 20 mpg saves 1.67 gallons per 100 miles, while going from 40 to 50 saves only 0.50. The smaller-looking upgrade at the thirsty end is worth over three times more.

Why is UK mpg higher than US mpg for the same car?

Because an imperial gallon is 4.546 litres against the US 3.785 — about 20% larger. The same car covers more miles on the bigger gallon, so a British 45 mpg is roughly a US 37 mpg. The figures are never directly comparable.

Is a lower L/100 km better or worse?

Better. Litres per 100 kilometres measures fuel used over a fixed distance, so less is good. It runs opposite to mpg, where more is good — which is exactly why the two systems confuse people.

Can I average two mpg figures?

Not by adding and halving. Driving 100 miles at 20 mpg and 100 at 40 mpg averages 26.7 mpg, not 30, because you burn 5 gallons on the first leg and 2.5 on the second. Convert to fuel per distance, average that, then convert back.

Why is my real consumption worse than the official figure?

Official figures come from a standardised lab cycle. Real driving adds cold starts, traffic, hills, wind, roof boxes, air conditioning and higher speeds. A gap of 10–20% is normal, and short winter journeys can be far worse.

Sources