Monday, February 8, 2010

Why GPM -- A Brief Review

Autobloggreen has a post on the New York State Senate bill that would require car dealers to describe fuel efficiency as "gallons per 1,000 miles". The post has prompted the familiar call for using the metric system (see this post on the connection between "GPM" and the metric system) and the familiar lament that people should understand the math.

A comment by "Throwback" on Autobloggreen reads:

Throwback 2:40PM (2/08/2010)

Another wasteful bill. What is the purpose? You don't think people understand that if they buy a car that gets 25 mpg vs 20 mpg they will be using less fuel? As a native New Yorker I am embarrassed by the (lack of) quality of the states politicians.

Here's a short answer to Throwback:

Yes, higher MPG is better than lower MPG, and people recognize this.

But people use the difference in MPG as the rough approximation of gas savings, which is misleading--often in a big way. Instead of subtracting MPG, car buyers need to first divide a given distance by each car's MPG, and then subtract. (See the last half of this post on the math.) That's what GPM does without effort--a standardized GPM measure can be subtracted to know actual gas savings.

Consider two trade ins:

A) 30 MPG to 45 MPG
B) 15 MPG to 20 MPG

Impressed by the 15 MPG improvement in option A? (A 50% improvement in MPG.)

Unimpressed by the 5 MPG improvement in option B? (A 33% improvement in MPG.)

Option A saves 11 gallons per 1000 miles; Option B saves 17 gallons per 1000 miles.

The New York bill is designed to highlight the gas savings available to people who are driving or considering cars in the teens. This is where the biggest savings are possible.

In fact, GPM may be most effective at keeping people who are currently driving more efficient cars from opting for less efficient ones. MPG tempts us to think that there is little harm in trading in a 20 MPG minivan for a 15 MPG SUV. What's 5 MPG? GPM makes clear the impact.

Compare MPG to "Gallons per 1000 miles" below:

MPG Gallons per 1000 miles
10 100
11 91
12 83
13 77
14 71
15 67
16 63
17 59
18 56
19 53
20 50
21 48
22 45
23 43
24 42
25 40
26 38
27 37
28 36
29 34
30 33
31 32
32 31
33 30
34 29
35 29
36 28
37 27
38 26
39 26
40 25
41 24
42 24
43 23
44 23
45 22
46 22
47 21
48 21
49 20
50 20