MPG (Miles Per Gallon) is a Stupid Metric

Give me my MPGs!  Everyone wants more MPGs because more is better.  By itself the unit is easy to understand; simply, it tells you how many miles you can go on one gallon.  It makes sense, but because of the nature of this ratio it causes some unusual misconceptions?

Answer the following:

  1. Does a five MPG improvement (from 20 to 25) give you the same gas (money) savings as a five MPG improvement (from 25 to 30)?
  2. Does comparing a SUV/Midsize Sedan (18 MPG versus 25 MPG) is the same as comparing a Midsize Sedan and Midsize Hybrid (25 MPG versus 32 MPG)?
  3. Does every time you improve one MPG you save the same amount of gallons/money?

Let’s assume the following:

  • On average you drive 12,000 miles a year.
  • The price for a gallon of gas is $3.339.

Question 1:

A 20 MPG car means you will use 600 gallons in a year.  (12,000/20=600)

  • 600 gallons at $3.339 a gallon will cost you $2,003.40

A 25 MPG car means you will use 480 gallons in a year.  (12,000/25=480)

  • 480 gallons at $3.339 a gallon will cost you $1,602.72
  • You saved $400.68 in gas by driving a 25 MPG car versus a 20 MPG car.

A 30 MPG car means you will use 400 gallons in a year.  (12,000/30=400)

  • 400 gallons at $3.339 a gallon will cost you $1,335.60
  • You saved $267.12 in gas by driving a 30 MPG car versus a 25 MPG car.

The higher the MPG the less incremental savings you gain by moving up one MPG.

Question 2:

A 18 MPG car means you will use 666.67 gallons in a year.  (12,000/18=666.67)

  • 666.67 gallons at $3.339 a gallon will cost you $2,226.00

A 25 MPG car means you will use 480 gallons in a year.  (12,000/25=480)

  • 480 gallons at $3.339 a gallon will cost you $1,602.72
  • You saved $623.28 in gas by driving a 25 MPG car versus a 18 MPG car.

A 32 MPG car means you will use 375 gallons in a year.  (12,000/32=375)

  • 375 gallons at $3.339 a gallon will cost you $1,252.13
  • You saved $350.59 in gas by driving a 32 MPG car versus a 25 MPG car.

Each car is 8 MPG better than the other but your savings moving from a mid-size sedan to a mid-size sedan hybrid is only half the savings moving from a SUV to a mid-size sedan.

Question 3:

  • Moving from a 9 MPG car to a 10 MPG car saves you $445.20 a year
  • Moving from a 39 MPG car to a 40 MPG car saves you $25.68 a year

Gallons per Thousand Miles (G/TM)

Most of the world uses “Liters per 100 Kilometers” or “L/100km” and this is the US unit equivalent.

  • 100 G/TM = 10 MPG (1,200 gal/year)
  • 50 G/TM = 20 MPG (600 gal/year)
  • 40 G/TM = 25 MPG (480 gal/year)
  • 30 G/TM = 33.3 MPG (360 gal/year)
  • 20 G/TM = 50 MPG (240 gal/year)

In this system, lower is better.  Assuming gas prices of $3.339 per gallon, each time you lower 10 G/TM you will save $400 a year.  This makes it very easy to compare the relative fuel costs of cars. 

Plug-In Vehicles

Now, how should we compare plug-in vehicles?  I don’t know.  Electricity costs vary so widely and do not fluctuate in tune with gas prices, so what is true today may not be true tomorrow.