Beware of MPG bringing gifts

Background

The standard measure for a vehicle's fuel efficiency in the US is MPG (Miles per Gallon). It represents how many miles you can drive this vehicle on one gallon of gas. Vehicles have two important MPG numbers: one for city driving and another for highway driving. It is also possible to compute a blended MPG number, as a function of how much you drive in the city vs. the highway. Here's an example from fueleconomy.gov for the 2013 Honda Civic:

In contrast, the standard measure for fuel efficiency in many European countries is "liters per 100 kilometers". It represents how many liters of fuel you need to drive this vehicle for 100km. For example, the 2013 VW Golf has an advertised fuel efficiency of 3.2 liters (of Diesel fuel) per 100 kilometers. 

MPG and liters/100km are interchangeable. Knowing that:

• 1 mile = 1.6 kilometers
• 1 gallon = 3.78 liters

The 2013 Honda Civic has a fuel efficiency of:

• 32 MPG = (32 * 1.6) / 3.78 = 13.54 km / liter
• To drive 100 km, you need 100 / 13.54 = 7.38 liters / 100 km

Conversely, the 2013 VW Golf has a fuel efficiency of:

• 3.2 liters / 100 km = (3.2 / 3.78) / (100 / 1.6) = 0.84 gallons / 62.5 miles
• This car will drive 74.4 MPG on one gallon of gas

Is there a reason to prefer MPG vs. liters/100km?

Suppose you want to upgrade your car to a newer model that burns less fuel. Which upgrade will result in greater fuel savings:

• (A) Going from a car with 18 MPG to a car with 20 MPG?
• (B) Going from a car with 30 MPG to a car with 32 MPG?

Choice (A) results in more than twice as much fuel saved than choice (B), even though the absolute MPG difference is the same (2 MPG), because the MPG varies inversely with gallons (and therefore cost).

 

Say you drive 10,000 miles in one year, and gas costs $4/gallon.

• Choice (A)
• At 18 MPG you spend: 10000 / 18 * 4 = $2222 on gas
• At 20 MPG you spend: 10000 / 20 * 4 = $2000 on gas
• Net savings: $222 (= 55 fewer gallons of fuel)
• Choice (B)
• At 30 MPG you spend: 10000 / 30 * 4 = $1333 on gas
• At 32 MPG you spend: 10000 / 32 * 4 = $1250 on gas
• Net savings: $83 (= 20 fewer gallons of fuel)

You save more than twice as much fuel by going from 18 to 20 MPG than by going from 30 to 32 MPG.

Now let's look at the same calculation in liters/100km:

• Choice (A)
• 18 MPG = 13.06 liters/100km
• 20 MPG = 11.76 liters/100km
• Net savings: 1.3 fewer liters of fuel/100km
• Choice (B)
• 30 MPG = 7.84 liters/100km
• 32 MPG = 7.35 liters/100km
• Net savings: 0.5 fewer liters of fuel/100km

In liters/100km, Choice (A) saves more than twice as much fuel than Choice (B), because liters/100km varies proportionally with gallons (and therefore cost).

Conclusion

Equal improvements in MPG do not translate to equal savings in fuel. Upgrading an 18 MPG car to a 20 MPG car makes a lot more sense than upgrading a 30 MPG car to a 32 MPG car, even though the difference is 2 MPG in both cases. The same goes for upgrading an 18 MPG car to a 19.8 MPG car vs. a 30 MPG car to a 33 MPG car, even though the difference is 10% in both cases.

Liters/100km is unambiguous about fuel savings. Upgrading a car from 8 to 7 liters/100km will result in the same fuel savings as upgrading a car from 15 to 14 liters/100km.

By the same reasoning, the city MPG number for a car matters more than the highway MPG number. A 2013 Honda Civic driven for 5000 miles in the city (at 28 MPG) and 5000 miles on the highway (at 39 MPG) will have an overall fuel economy of 32, which is a lot closer to the city number than to the highway number.

If your goal is to reduce fuel consumption by purchasing a more efficient car, be sure to convert from MPG to liters/100km in order to get a more accurate picture!