The Importance of Units

Wednesday, May 7, 2008 at 2:23 PM

Posted by Adam Langley

Fuel economy is generally measured, in the US, in terms of miles per gallon. The average US car drives about 12,000 miles in a year. Here's a graph of fuel used against fuel economy:






Obviously, if your car does one mile per gallon it will take 12,000 gallons to drive 12,000 miles. However, we're also marked a few other points on the graph to highlight the counter-intuitive behaviour of a reciprocal curve.

You can see that getting a 10 MPG car to do 12 MPG saves as much fuel (1200 - 1000 = 200 gallons) as switching a 30 MPG car for a 60 MPG one (400 - 200 = 200 gallons). This result surprises many people.

If we were to use gallons per 100 miles as our unit of fuel economy, then the graph would be a straight line. Now, 10 MPG is 10 G/100M and 12 MPG is 8.3 G/100M, a difference of 1.7. Also 30 MPG is 3.3 G/100M and 60 MPG is 1.7G/100M, again a difference of 1.6. Now it's immediately obvious that the fuel savings are the same.

This style of unit is found in many European countries, among others; although they use litres per 100 kilometers. These units give a much better indication of fuel use.

We already have fuel economy in gallons per 100 miles and litres per 100 kilometers for the headline numbers on our website and we'll be making an effort in the coming weeks to use them in more places.