Fuel Usage and Cost for Various MPG Ratings

The actual fuel usage and therefore CO2 pollution is not given by the miles per gallon rating (mpg) but by its inverse, the gallons per mile.  There are about 20 pounds of CO2 created for every gallon burned.  For several values of mpg, we invert to gallons per mile, and we multiply the gallons per mile by 300 miles.  That is an average weekly driving by a commuter who drives 60 miles roundtrip to work five days a week.  For 50 weeks of driving it gives 15,000 miles per year, a high but possible figure for commuters.  Gas tanks typically hold fuel for over 300 miles, but you may typically refuel them when they start getting low on fuel at that distance.  Now that gas is heading toward $4 a gallon in California, I will use that price in adding a third column to the table for cost per refill.

So the first column of the table is mpg by 5’s from 10 mpg (Hummer) to 60 mpg (55 mpg is the 2025 future mileage standard).  The second column is the inverse in gallons per mile for 300 miles.  The third column is the price of the fill-up after 300 miles at $4 per gallon.  Since tables are hard to make and align or insert in WordPress, I will just make it by spacing.

MPG          Gallons      Cost ($)

for 300 miles

10                    30        120

15                    20        80

20                    15        60

25                    12        48

30                    10        40

35                    8.6       34.2

40                    7.5       30

45                    6.7       26.7

50                    6          24

55                    5.5       21.8

60                    5          20

The current average car mileage is about 27 mpg.  On sees in this table the law of diminishing returns.  As one pushes toward ever higher mpg, the diminution in gallons used or weekly cost of gas is very small.  There are gas only cars now that claim 35 mpg at highway speeds.  If one pays several thousand extra for an electric hybrid that promises 45 mpg, the savings are only about $7.50 a week or $390 a year.  You only save about two gallons of gas a week.  The real key to lowering vehicle CO2 emissions is to eventually hybridize heavy vehicles, or talk people out of very heavy cars getting only 15 or 20 mpg, which burn 80 to 60 gallons a week.  The decade away mpg standard of 55 mpg is only for the smallest cars.  Compared to the Prius hybrids that already get 45 mpg, only save 1.2 gallons a week, or about $5 a week.  For these economical cars, the future is already here.

About Dennis SILVERMAN

I am a retired Professor of Physics and Astronomy at U C Irvine. For two decades I have been active in learning about energy and the environment, and in reporting on those topics for a decade. For the last four years I have added science policy. Lately, I have been reporting on the Covid-19 pandemic of our times.
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