MPG

Miles per gallon (MPG) is a misleading quantity.

Miles per gallon is often referred to as a measurement of fuel consumption but it isn’t. Miles per gallon is a measure of the car’s fuel efficiency: how many miles the car can extract from one tank of fuel. A more efficient car can extract more miles from a tank.

Which saves you more fuel?

• Switching from a car that gets 20MPG to one that gets 40MPG.
• Switching from a car that gets 40MPG to one that gets 60MPG.

For a hypothetical 400 mile trip:

• Switching from 20MPG to 40MPG saves 10 gallons of fuel.
• Switching from 40MPG to 60MPG saves 3.3 gallons of fuel.

As fuel efficiency increases the amount of fuel saved decreases. A 20MPG increase in the low MPG values has far more of an effect than a 20MPG increase in the high MPG values. The scale is non-linear.

Put another way: the average MPG rating for a car is about 27MPG. For our hypothetical 400 mile trip:

• Doubling fuel efficiency to 54MPG saves a total of 7.4 gallons of fuel.
• Tripling fuel efficiency to 81MPG saves a total of 9.9 gallons of fuel.

This dependence on MPG is (yet another) hangover from our archaic insistence on non-standard imperial units.

l/100km

To measure fuel consumption, we need to measure not miles per gallon, but gallons per mile. A (volume) per (distance) measurement already exists and is already commonly used in Europe: litres per 100 kilometres.

Which saves you more fuel?

• Switching from a car that gets 6l/100km to one that gets 4l/100km.
• Switching from a car that gets 4l/100km to one that gets 2l/100km.

For a hypothetical 400km trip:

• Switching from 6l/100km to 4l/100km saves 8 litres of fuel.
• Switching from 4l/100km to 2l/100km saves 8 litres of fuel.

In this case the scale is linear, a 2l/100km saving always results in the same fuel saving.

TL;DR

We should use the l/100km rather than the MPG scale for measuring fuel consumption.