CAFE math

From Marginal Revolution:

Via Andrew Sullivan, Eric dePlace notes that "You save more fuel switching from a 15 to 18 mpg car than switching from a 50 to 100 mpg car." And so you do. A 15 MPG car would require 1,000 gallons of gas to drive 15,000 miles while an 18MPG car could get it done in just 833 gallons. That saves 167 gallons of gasoline. By contrast, since a 50 MPG only uses 300 gallons to go 15,000 miles, upgrading to 100 MPG can't save that much gas -- the super-efficient car uses 150 gallons.

It's called "diminishing returns."  It's easy to see if you chart the number of gallons saved by improving a given gas mileage by 1 mpg, which I've done below (per 1,000 miles driven).

 

Improving gas mileage from 10 mpg to 11 mpg saves 9 gallons of gas per 1,000 miles (all else equal), while improving gas mileage from 50 mpg to 51 mpg saves just 0.4 gallons per 1,000 miles -- or just 4 gallons per year, if you drive 10,000 miles per year.

I think CAFE is a dumb piece of legislation for a bunch of reasons.  One is its focus on "fleet averages."  CAFE orders car makers to improve the average fleet gas mileage.  A car maker can do that by improving the fuel efficiency of its clunky SUVs.*  Or it can do that by improving the fuel efficiency of its already-efficient compacts.  (It can do both, of course.)  Increasing a 35 mpg vehicle's gas mileage by 10 mpg saves just 10% as much fuel as increasing a 15 mpg vehicle's gas mileage by 10 mpg.  These are perfectly equivalent methods of compliance under CAFE, though, so a car maker will choose whichever option (or combination of improvements) will cost it the least.

If you think this kind of legislation is a good thing, there's a simple fix:  mandate reduction in average "gallons per mile."  That would give a 1 mpg increase in a low-gas-mileage vehicle a lot more weight than a 1 mpg increase in a hybrid's gas mileage.  E.g., improving gas mileage from 15 mpg to 20 mpg is a 25% reduction in "gallons per mile," but improving gas mileage from 40 mpg to 45 mpg is just an 11% reduction in gallons per mile.  The car maker would have an incentive to make the improvements where they would make the most difference.

*Under the new CAFE standards, car makers will have to lump their SUVs with their passenger cars in calculating fleet averages.

Comments

[Heyzeus]

CAFE used to have an exemption for "heavy trucks," which as defined included most pickups and SUVs. I'm hoping that this time around, Congress had the wisdom to redefine this term so that standard consumer-bound pickups and SUVs are included.

[M1EK]

heyzeus,

It's not as bad as old CAFE, but new CAFE has a ton of exemptions and loopholes to keep Detroit pumping out SUVs for a long time.

AC, the reason this battle had to be fought is that so many self-identified libertarians have somehow gotten the idea that Detroit was just "making what people wanted to buy" when they exploited the old CAFE loophole for SUVs (destroying most of the station wagon market in the meantime). Don't forget, also, that these same people are deeply and firmly against a gasoline tax, tolls, and anything else that might actually have more of an effect than CAFE does.

[Garko Novis]

buying gas has become a major investment decision, as in "do i invest in some food so i can get thru the day or some gas so i can get where i have to go?" It should never be this way but it is. But that doesn't mean we have to just suffer. There is a real solution in Water4Gas and you owe it to yourself to check it out! http://w4g4mpg.info

[Mike]

My Lexus LS400 went from 19 to 30mpg. After I purchased Water for gas I was impressed with the design. Then when I purchased all of the parts the size of the hoses and lack of flow of the check valves concerned me. Being a Machine Designer as well as an Electro-Mechanical and Pneumatic assembler I took the liberty of redesigning the devise a bit. Anyway I already have the parts list specked out for these changes. I'll give them out for free.