Cash for Clunkers and the GPM Calculator

The GPM calculator has now been updated with all MPG levels from 10 to 30 MPG (and all even MPG values from 30 to 50 MPG).

This tool is ideal for letting you see your gas and cost savings from a "Cash for Clunkers" trade in.

How to use it: Select a driving distance and gas price of your choice. Compare your current MPG level (15 MPG) to a wide range of more efficient MPG levels (e.g., 24 MPG, 27 MPG, 33 MPG) to see your gas and cost savings.

You might also find the following tables helpful:

1) These tables directly caclulate the gas and cost savings of trading in a car that gets 18 MPG or less for different combinations of cars with higher MPG.

2) The following table shows why MPG is misleading as a measure of gas savings and why we need to look at "GPM" measures such as "Gallons per 100 Miles". "Gallons per 100 miles" makes clear that the improvement from 14 MPG to 25 MPG saves more gas (3 gallons per 100 miles) than any possible improvement on a 33 MPG car. Here is a downloadable version of the table below.

MPG	Gallons per 100 Miles	Gas Costs to Drive 10,000 Miles
10	10.0	$2,500
11	9.1	$2,273
12	8.3	$2,083
13	7.7	$1,923
14	7.1	$1,786
15	6.7	$1,667
16	6.3	$1,563
17	5.9	$1,471
18	5.6	$1,389
19	5.3	$1,316
20	5.0	$1,250
21	4.8	$1,190
22	4.5	$1,136
23	4.3	$1,087
24	4.2	$1,042
25	4.0	$1,000
26	3.8	$962
27	3.7	$926
28	3.6	$893
29	3.4	$862
30	3.3	$833
31	3.2	$806
32	3.1	$781
33	3.0	$758
34	2.9	$735
35	2.9	$714
36	2.8	$694
37	2.7	$676
38	2.6	$658
39	2.6	$641
40	2.5	$625
41	2.4	$610
42	2.4	$595
43	2.3	$581
44	2.3	$568
45	2.2	$556
46	2.2	$543
47	2.1	$532
48	2.1	$521
49	2.0	$510
50	2.0	$500

If you want to know your carbon impact, saving 100 gallons of gas reduces carbon dioxide emissions by roughly 1 ton.

[Update June 20: Here is a link to the final June 18 plan.]

Carbon Pay Back Period and Cash for Clunkers

[Update June 20 2009

If you are considering a trade in, please use the tools linked here to see the cash value of your gas savings. The gas savings can be more valuable than a Cash for Clunkers voucher.

The final bill is linked to this post. The details below refer to an older bill.]

Bill Chameides, dean of Duke's Nicholas School, has a nice post on Cash for Clunkers at his Green Grok blog (see also this Christian Science Monitor article and Huffington Post summary):

http://www.nicholas.duke.edu/thegreengrok/cashforclunkers

He provides data showing that the CO2 emissions from manufacturing a new vehicle are close to 7 tons on average, which is a higher figure than I was assuming in earlier posts. That makes the CO2 "pay back" period longer when replacing an old car with a new one. Although my favorite example of replacing a 14 MPG car with a 25 MPG car holds up fine (the carbon emissions are offset in less than 25,000 miles of driving), the case for smaller improvements becomes much harder.

The New York Times reviews the two bills currently circulating: The Israel/Schumer/Feinstein bill ensures a sizeable increase in GPM (roughly from the mid teens to the high 20s) that quickly "pays" for itself in CO2 reduction; the Sutton and Stabenow bills do not.

The key insight: Ideally, Cash for Clunkers rebates would be tied to the magnitudue of GPM improvement. The goal would be to reimburse a 14 to 25 MPG replacement more highly than a 24 to 27 MPG replacement (the first change reduces CO2 by 3 tons over 10,000 miles, the second by just half a ton). If reimbursements are not tied directly to GPM improvements, then the focus should be on rewarding trading in cars in the teens for cars in the high 20s (as emphasized in the Israel/Schumer/Feinstein bill).

USA Today adds Gallons per 100 Miles

James Healey, the auto writer for USA Today, has added gallons per 100 miles to his car reviews.

In his Prius review, he writes: "How thirsty? Rated 51miles per gallon in town, 48 highway, 50 in combined driving. That's about 2 gallons per 100 miles, using the gallons-per-mile index that's emerging as a better measure of fuel use and cost. Trip computers in preproduction test cars showed: 56.2 mpg in suburbs (1.78 gallons per 100 miles); 48 mpg highway (2.08 gal./100 miles)."

In a chat session, a curious reader asked Healey: "I noticed the peculiar gas mileage listing in the Prius article, 1.78 gallons per 100 miles. What's that about?"

Healey replies, "It is, some folks argue pretty effectively, a crisper and more relevant way to compare vehicles and get a quick handle on the difference in fuel use and cost. A hybrid might use 2 gallons per 100 miles ($4 at today's prices) and a gasoline economy car might use 3 gal./100 mi ($6). You pretty quickly can decide from those numbers if the price premium for a hybrid is worth saving 1 gal. ($2) every 100 miles. Or take it out 1,000s -- 20 gallons ($40) for 1,000 mi. vs. 30 gal. ($60) per 1,000 mi. It's a takeoff of the Euro system, where fuel consumption is stated in liters per kilometer. Because the distance usually is fixed -- you drive 300 miles a week, week-in, week-out, for instance -- you can get a fast notion of consumption and cost."

Cash for Clunkers: Tables for Car Buyers and Car Dealers

Update June 20, 2009

The final June 18 Cash for Clunkers bill is described here.

Follow this link to an online GPM calculator and a downloadable table of gas savings based on the MPG levels in the new bill. See the blog archive on the right to find recent posts on Cash for Clunkers.

The specific information that was posted below is now outdated and we have deleted it. Please consult the new post.