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MPG vs. GPM
Posted by W2GMiles Per Gallon (MPG) vs. Gallons Per Mile (GPM):
By: Danny S.
Quiz time – Is it more economical (from a cost of fuel standpoint) to replace a vehicle that gets 14 MPG with a vehicle that gets 18 MPG, or replace a vehicle that gets 28 MPG with a vehicle that gets 40 MPG?
By looking at MPG alone, one might think that the 12 MPG improvement gained by replacing the 28 MPG vehicle would yield higher savings than the 4 MPG improvement gained by replacing the 14 MPG vehicle. The catch here is that MPG increases are non-linear. Looking at both the percentage increases and the means of the two vehicle combinations also leads to an economic misinterpretation. The first scenario leads to a 22% improvement in MPG while the second scenario leads to a 30% improvement.
Using the basic national average of 12,000 miles per year, the 14 MPG vehicle would use approximately 857 gallons of gas per year and the 18 MPG vehicle would use approximately 667 gallons of gas per year, resulting in a reduction of 190 gallons of gas per year. On the other hand, the 28 MPG vehicle would use approximately 429 gallons of gas per year and the 40 MPG vehicle would use approximately 300 gallons of gas per year, providing a difference of 129 gallons per year. The annual difference between the two vehicle scenarios is 61 gallons per year (190 - 129). At $4.20 per gallon, that is a yearly savings of approximately $256. At 20 lbs of CO2 per gallon of gas, that is a reduction of 1,220 lbs of CO2 per year. The idea here is that a larger fuel savings is seen by replacing low averaging MPG vehicles, even if it is only by a few MPG, than replacing high averaging MPG vehicles. This could be the situation of a family that has an SUV and a sedan. They may think that replacing the sedan with a hybrid would lead to the better economic choice because of the larger MPG reduction. However, as we have seen they would be better off replacing the SUV with a minivan, large sedan, or station wagon.
In a recent article, Duke University Professors Richard Larrick and Jack Soll produced a study showing that rating vehicles by gallons per mile (GPM) rather than MPG leads to more accurate choices. A simple rating of gallons per 100 miles or per 10,000 miles would show exactly how much of a fuel savings is gained or lost when comparing two vehicles. In our scenario above, the 14 MPG vehicle uses approximately 7.1 gallons per 100 miles, while the 18 MPG vehicle uses approximately 5.6 gallons per 100 miles. In the second scenario, the 28 MPG vehicle uses approximately 3.6 gallons per 100 miles, while the 40 MPG vehicle uses approximately 2.5 gallons per 100 miles. If this information were listed on a vehicles spec sheet you would simply subtract the gallons per mile from one another (7.1 – 5.6 = 1.5).
Let me ask the question again – Is it more economical (from a cost of fuel standpoint) to replace a vehicle that uses 7.1 gallons per 100 miles with a vehicle that uses 5.6 gallons per 100 miles, or replace a vehicle that uses 3.6 gallons per 100 miles with a vehicle that uses 2.5 gallons per 100 miles?
The original article and charts can be found at Science Magazine (subscription required)
For addition research from Duke University
|
MPG |
Gallons used/yr |
Gallons saved/yr |
Savings @ $4.20/gal |
CO2/yr |
CO2 saved/yr |
GPM (gal/100 miles) |
GPM saved(gal/100 miles) |
|
14 |
857 |
190 |
$798 |
17,140 |
3,800 |
7.14 |
1.58 |
|
18 |
667 |
13,340 |
5.56 |
||||
|
28 |
429 |
129 |
$542 |
8,580 |
2,580 |
3.57 |
1.07 |
|
40 |
300 |
6,000 |
2.50 |
