Is that really true?
Does better gas mileage really save much money?
The Simple Answer
It depends where you start. The first few miles per gallon save the most. Going from 15 to 20 mpg saves more gas than going from 30 to 50.
This surprises people because miles per gallon sounds like a straight scale. It is not. What you pay for is gallons, and gallons are miles divided by mpg, which curves.
A driver who covers 12,000 miles a year
At 15 mpg: 12,000 ÷ 15 = 800 gallons
At 20 mpg: 12,000 ÷ 20 = 600 gallons, so 200 gallons saved
At 30 mpg: 12,000 ÷ 30 = 400 gallons
At 50 mpg: 12,000 ÷ 50 = 240 gallons, so 160 gallons saved going from 30
Five extra miles per gallon on a thirsty car beat twenty extra on an efficient one, 200 gallons against 160.
| Change | Gallons used | Gallons saved | Money kept at $3.50 |
|---|---|---|---|
| 10 to 15 mpg | 1,200 to 800 | 400 | $1,400 |
| 15 to 20 mpg | 800 to 600 | 200 | $700 |
| 20 to 25 mpg | 600 to 480 | 120 | $420 |
| 25 to 30 mpg | 480 to 400 | 80 | $280 |
| 30 to 40 mpg | 400 to 300 | 100 | $350 |
| 40 to 50 mpg | 300 to 240 | 60 | $210 |
| 50 to 60 mpg | 240 to 200 | 40 | $140 |
Where the idea comes from
In 2008, two researchers, Richard Larrick and Jack Soll, published a paper in Science called "The MPG Illusion". They found that people wrongly assume the gas a car uses falls steadily as its mpg rises, when the real relationship curves. They suggested describing fuel economy as gas used per distance, such as gallons per 100 miles, so that the savings are easy to see. The U.S. fuel economy label now shows consumption alongside mpg.
What it means when you shop
- Replacing a very thirsty vehicle with a middling one can save more than replacing a middling one with a very efficient one.
- The more you drive, the more every step is worth. Halve the miles and you halve every saving in the table.
- Gas is only part of the cost of owning a car. The table says nothing about the price of the car, insurance or repairs.
Gas vs EV Cost Comparison: Compare the yearly running cost of a gas car and an electric one from your own numbers.
Trip Cost Estimator: Work out what a trip costs in fuel from the distance, your mpg and the price of gas.