Nicholas School Dean Bill Chameides proposes a tougher pay back rule than our proposal of 1 GPM: He proposes that any new car needs to pay back the carbon debt in about 2.3 years of driving at 13,000 miles per year. Put somewhat differently, this requires a payback over 30,000 miles of driving to be carbon neutral. (We proposed that a new car pay back over 70,000 miles, which, at 13,000 miles per year, is a maximum of 5.4 years).
Dean Chameides has prepared some nice graphs at his Green Grok site:
This cool table translates to something like the following rule for old and new cars:
12 MPG must be raised to at least 17 MPG
13 MPG must be raised to at least 18 MPG
14 MPG must be raised to at least 21 MPG
15 MPG must be raised to at least 23 MPG
16 MPG must be raised to at least 26 MPG
17 MPG must be raised to at least 28 MPG
18 MPG must be raised to at least 31 MPG
.... to merit a voucher.
Of course, because of the non-linear relationship between CO2 reduction and MPG, there is no simple rule about MPG increase that captures these changes. Let's translate it to GPM, which is linear in CO2.
The Green Grok standard equates to a 2.5 GPM principle: A new car needs to save 2.5 gallons of gas every 100 miles compared to the original car to be carbon neutral.
I would add, as in earlier posts, that a tiered system that increased the voucher size directly with the reduction in GPM makes the most sense. Let's have more voucher levels that increase as GPM shrinks. [Update: See this May 10 post for a tiered system based on GPM (not MPG!). This post links to the final June 18 Cash for Clunkers bill.]
Carbon Payback Period and Gas Savings
This table shows the gas (and cost) savings from trading in a low MPG car for a higher MPG car (available at this post). The savings are calculated over 10,000 miles of driving. As you look at our table, keep in mind that reducing gas consumption by 100 gallons saves a ton of CO2. One must save 700 gallons of gas to "pay back" the carbon dioxide emissions when manufacturing a new car.
Finally, note the (necessary) connection between our gas savings table and Dean Chameides' pay back table. The MPG combinations that yield a 2.3 year pay back in his table are the same as our MPG combinations that yield gas savings of roughly 240 gallons over 10,000 miles, or 2.4 gallons per 100 miles.