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:
- 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)?
- 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)?
- Does every time you improve one MPG you save the same amount of gallons/money?
- On average you drive 12,000 miles a year.
- The price for a gallon of gas is $3.339.
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
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.
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