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

Gallons per Mile Bill Clears New York Senate Committee

[Update April 21: The GPM bill passed the New York Senate yesterday as part of larger Earth Day legislative package. See this post for details. The original bill, described below, required that dealers provide "gallons per 1,000 miles" for each vehicle they sold. The new bill requires that dealership display a chart translating mpg to "gallon per 1,000 miles".]

The New York Senate Environmental Conservation Committee has passed a new fuel efficiency bill that includes a "gallons per mile" requirement. The bill requires that vehicle manufacturers list "gallons per 1,000 miles" for city, highway, and combined driving.

The bill was championed by Senator Daniel Squadron, who laid out his rationale in this December article, and received broad support from environmental groups:

"Urging the passage of Senator Squadron’s bill were the New York League of Conservation Voters, the Environmental Defense Fund, the Natural Resources Defense Council, the Sierra Club’s Atlantic Chapter, and Senator Antoine Thompson, the Chair of the Environmental Conservation Committee."

The blog All over Albany lays out the case in more detail.

We believe this low cost change for presenting fuel efficiency information is of great value to consumers and the environment. We applaud Senator Squadron for endorsing it.

Although we advocated for "gallons per 100 miles" and "gallons per 10,000 miles" as useful standards in the supplement to the Science article, we note the benefits of "gallons per 1,000 miles":

1. 1,000 miles is roughly what the average American drives in a month, so it is a meaningful number
2. It allows easy estimation of yearly consumption (multiply by 10, roughly)
   
3. It avoids the problem of seemingly small differences in efficiency that occurs when comparing "gallons per 100 miles"

Barry Nalebuff and Ian Ayres made the case for gallon per 1,000 miles in this Why Not? column for Forbes magazine.

Here is an excerpt from the bill (the full bill can be found here):

BILL NUMBER: S6141

TITLE OF BILL :
An act to amend the environmental conservation law, in relation to requiring automobile dealers display a fuel economy label on all new vehicles

PURPOSE :
To require that new passenger motor vehicles sold in New York State post a "gallons-per-mile fuel impact statement."

SUMMARY OF PROVISIONS :
Adds a new section 19-1104 to the environmental conservation law to require vehicle manufacturers to display a gallons per thousand miles fuel impact statement. Such statement shall set forth the average number of gallons the vehicle is expected to use when traveling a distance of one thousand miles of city mileage, highway mileage and combined city and highway mileage. The bill provides for a civil fine of not more than $100 per vehicle to be imposed on manufacturers for a violation of this section.


JUSTIFICATION :
Gallons per miles driven is a much more useful means of measuring fuel efficiency than the current miles per gallon standard. It enables a vehicle purchaser to more easily compare the fuel efficiency between various models of automobiles. This bill, which would require manufacturers to display the average gallons per one thousand city, highway and combined miles, will allow consumers to know accurately at a glance the cost of operating a vehicle over one thousand miles. Additionally, this allows environmentally conscious consumers to identify the relative environmental effect of different vehicles. Encouraging consumers to buy more fuel efficient vehicles can help save money, reduce greenhouse gas emissions, improve energy security and oil dependence costs, and increase energy sustainability.