Saturday, October 25, 2008

Tools for Calculating GPM from MPG


For an online GPM calculator that includes conversions for all new 2009 cars, go to this post
This May 29 post has several tables that can help you calculate the gas savings from a Cash for Clunkers trade in.

Original Post

The following table shows how different levels of MPG translate to gallons of gas consumed over 10,000 miles (GPM). The MPG levels are chosen so that they translate to equal improvements in gas saving (100 gallons over 10,000 miles):

10.0 MPG = 1000 GPM (Gallons per 10,000 Miles)
11.0 MPG = 900 GPM
12.5 MPG = 800 GPM
14.0 MPG = 700 GPM
16.5 MPG = 600 GPM
20.0 MPG = 500 GPM
25.0 MPG = 400 GPM
33.0 MPG = 300 GPM
50.0 MPG = 200 GPM

The table makes clear that small MPG improvements on inefficient cars (e.g., 11 to 12.5, 14 to 16.5) save a large amount of gas. Replacing a 14 MPG car with a 25 MPG car saves more gas over a given distance than any possible improvement to a 33 MPG car. Greenhouse gas policy needs to focus on removing the most inefficient cars.

The following table shows how improvements of 5 MPG translate to gas consumption (gallons per 10,000 miles):

10 MPG = 1000 GPM (Gallons per 10,000 Miles)
15 MPG = 666 GPM
20 MPG = 500 GPM
25 MPG = 400 GPM
30 MPG = 333 GPM
35 MPG = 285 GPM
40 MPG = 250 GPM
45 MPG = 222 GPM
50 MPG = 200 GPM

This table makes clear the diminishing marginal returns to higher MPG.

These tables are summarized in this pdf file. Print it and keep it with you to calculate gas consumption when buying a new car.

An online GPM calculator can be found at this post. It also contains calculators for all new 2009 cars.

If you are interested in making GPM calculations for any distance of your choice, these excel files will do the math for you. They open in a new window and can be downloaded and saved to your computer:

GPM calculator for one car

GPM calculator for two cars

The second file (for two cars) is useful for (1) comparing the difference in efficiency between two cars, in which case one would use a single distance for both cars, and for (2) comparing total gas consumption for two cars in the same household, in which case one can let the distances of both cars vary to match expected driving.

If you find graphs helpful, the following picture shows the amount of gas used (on the y-axis) for different levels of MPG (shown as different lines) and for different distances of driving (on the x-axis). Click on the icon below to go to a two-page pdf file (click here for the pdf or here for a powerpoint file). The first page contains MPG values that range from 10 to 50; the second page contains MPG values for (relatively) efficient cars that range from 20 to 50.

Burning one gallon of gas releases about 20 pounds of carbon dioxide (another 20 percent is released in producing gasoline). Every 100 gallons saved reduces carbon dioxide emissions by 1 ton.

Other calculators can be found at these sites (we did not construct these calculators and have not used them extensively): (which is linked to the July 14 post at good republican usa)