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