Fig. 20Conversion scale and tank after the trip

The two scales run in opposite directions: litres per 100 km measures fuel per distance, mpg measures distance per fuel. On a logarithmic rule they line up exactly, which is how a slide rule would do this conversion.

§ How it works

Europe measures how much fuel a fixed distance costs you (L/100 km, lower is better). The US and UK measure how far a fixed amount of fuel takes you (mpg, higher is better). They are reciprocals of one another, scaled by unit conversions.

\text{mpg}_{\text{US}} = \frac{235.215}{\text{L/100 km}} \qquad\qquad \text{mpg}_{\text{UK}} = \frac{282.481}{\text{L/100 km}}

The constant comes from $100 \times 3.78541 / 1.609344 = 235.215$ : litres in a US gallon and kilometres in a mile. The imperial gallon is 20 % larger (4.546 L), so UK mpg figures always look better than US ones for the same car.

Trip fuel and cost

V = \frac{d \times c}{100} \qquad\qquad \text{cost} = V \times p

$d$ : distance in km. $c$ : consumption in L/100 km. $p$ : price per litre.
Range on a full tank is $100 \times V_{\text{tank}} / c$ .

Why mpg is misleading

Because mpg is a reciprocal, equal steps do not save equal fuel. Going from 15 to 20 mpg saves 3.9 L every 100 km. Going from 40 to 45 mpg saves only 0.65 L. Litres per 100 km is linear, so it shows savings directly.

Carbon dioxide

Burning fuel turns its carbon into CO₂ at a fixed rate: about 2.31 kg per litre of petrol and 2.68 kg per litre of diesel.

m_{\text{CO}_2} = V \times 2.31\ \text{kg/L}

References

Greenhouse Gas Emissions from a Typical Passenger Vehicle — US EPA8,887 g CO₂ per gallon of gasoline (≈ 2.35 kg/L)
Larrick & Soll (2008), “The MPG Illusion”, Science 320(5883) — doi:10.1126/science.1154983original paper on why miles-per-gallon misleads
Gallons Per Mile Would Help Car Shoppers — Duke Fuqua School of Businessplain-language summary of the MPG-illusion experiments

Fig. 20Conversion scale and tank after the trip

§ How it works

\text{mpg}_{\text{US}} = \frac{235.215}{\text{L/100 km}} \qquad\qquad \text{mpg}_{\text{UK}} = \frac{282.481}{\text{L/100 km}}

Trip fuel and cost

V = \frac{d \times c}{100} \qquad\qquad \text{cost} = V \times p

$d$ : distance in km. $c$ : consumption in L/100 km. $p$ : price per litre.
Range on a full tank is $100 \times V_{\text{tank}} / c$ .

Why mpg is misleading

Carbon dioxide

Burning fuel turns its carbon into CO₂ at a fixed rate: about 2.31 kg per litre of petrol and 2.68 kg per litre of diesel.

m_{\text{CO}_2} = V \times 2.31\ \text{kg/L}

References

Greenhouse Gas Emissions from a Typical Passenger Vehicle — US EPA8,887 g CO₂ per gallon of gasoline (≈ 2.35 kg/L)
Larrick & Soll (2008), “The MPG Illusion”, Science 320(5883) — doi:10.1126/science.1154983original paper on why miles-per-gallon misleads
Gallons Per Mile Would Help Car Shoppers — Duke Fuqua School of Businessplain-language summary of the MPG-illusion experiments

Fuel Economy & Trip Cost

Inputsdrag · tap · type

Fig. 20Conversion scale and tank after the trip

§ How it works

Trip fuel and cost

Why mpg is misleading

Carbon dioxide

References

Fuel Economy & Trip Cost

Inputsdrag · tap · type

Fig. 20Conversion scale and tank after the trip

§ How it works

Trip fuel and cost

Why mpg is misleading

Carbon dioxide

References