Gallons per Hundred Miles or GPHM

Have a look at this New York Times Year in Ideas article, which talks about a new and better way to measure the fuel economy of our vehicles. They suggest we use gallons of gas used to go a hundred miles instead of the traditional miles per gallon. The old way (MPG) tells us how far we can go on a certain amount of gas. The new way (GPHM) tells us how much gas we use to go a certain distance. So why the change? What is wrong with the old way?

Well, there is nothing wrong with it, but it can be very misleading. Basically the old system (MPG) is set up for calculating the range of a vehicle given the amount of gas available. You multiply the amount of gas you have by the MPG number to get the miles you can go. But in the world of fuel economy, we want to ask the opposite question. We want to know how much gas a vehicle will use to go a known distance. Using MPG as a fuel economy rating can give surprising results.

For example, consider a family (call them the Jones') that has a Ford F150 pickup truck that gets 10 miles per gallon. Their neighbors (the Smiths) have a Prius that gets 40 miles per gallon. We pretend that the two families drive these vehicles exactly 10 000 miles in the year 2008. How much gas did each family use up? The Jones' burned through 1000 gallons of gas while the Smiths used 250 gallons.

Okay, now imagine that each family tries to improve the gas mileage of their vehicle. They maintain the tire pressure at the correct value, replace the air filters and have a four-wheel alignment done. As a result, they each enjoy a 3 MPG increase in mileage. Now the Jones' get 13 MPG with their F150, and the Smiths get 43 MPG. In 2009 they again each put 10 000 more miles on their vehicles. How much gas did they burn up in 2009? The Jones family used 769 gallons while the Smith family used 233 gallons. Year over year, the Jones family saved 231 gallons but the Smith family only saved 17 gallons!

This is the problem with using miles per gallon (MPG) to measure fuel economy: the same increase in MPG (in our example 3 MPG more) does not mean the same amount of gas saved. The amount of gas saved depends on both the MPG number and the change in the MPG. Having to think about both the number and the change is unnecessarily complex. Using gallons per hundred miles lets us use just one number again. Let's see the example again, but this time using GPHM.

In 2008, the Jones' Ford F150 used 10 GPHM, and the Smith's Prius used 2.5 GPHM. After the mechanical tuneups, in 2009 the numbers were 7.7 GPHM for the Jones' and 2.33 GPHM for the Smiths. Now you can see the big drop (2.3 GPHM) for the Jones' compared to the tiny (0.17 GPHM) gain made by the Smiths.

So think about fuel economy in gallons per hundred miles (GPHM) instead of miles per gallon (MPG) and you will have a better idea of what is really going on!

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