An increase in fuel economy by 5 miles per gallon (mpg) does not translate to a constant fuel savings amount. Thus, trading a low-mpg car or truck for one with just slightly better mpg will save more fuel than trading a high-mpg car or truck for one that is even higher. For example, trading a truck that gets 15 mpg for a new one that gets 20 mpg will save 16.7 gallons of fuel for every 1,000 miles driven. In contrast, trading a 35 mpg car for a new car that gets 40 mpg will save 3.6 gallons of fuel for every 1,000 miles driven. These fuel savings are additive; that is, going from 15 mpg to 25 mpg saves 26.7 gallons per thousand miles driven — 16.7 gallons (15-20 mpg difference) plus 10.0 gallons (20-25 mpg difference).

Fuel Savings per Thousand Miles
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Supporting Information

If you have a car with this fuel economy...And you trade it for a car with this fuel economy...You save this many gallons for every 1,000 miles you drive
5 mpg10 mpg100.00
10 mpg15 mpg33.3
15 mpg20 mpg16.7
20 mpg25 mpg10.0
25 mpg30 mpg6.7
30 mpg35 mpg4.8
35 mpg40 mpg3.6
40 mpg45 mpg2.8
45 mpg50 mpg2.2
50 mpg55 mpg1.8
55 mpg60 mpg1.5

Source:
U.S. Department of Energy and Environmental Protection Agency, Fuel Economy Guide Website.

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