The Time is Ripe to Green Trucking (and why MPG is inverserly propotional)
Posted Oct 27 2009 11:00pm
Due to the logarithmic nature of fuel efficiency there is more to be gained to green trucks than work on small cars.
For 50 years, long haul tractor-trailer designs have remained fundamentally unchanged. Basically a giant box hurtling down the highway at 55 miles per hour, most trucks average only six miles to the gallon.
But the time is ripe for change. According to recent analysis by Rocky Mountain Institute the technology already exists to double the energy efficiency of long-haul trucks in the nation’s fleet. Their size, speed and poor aerodynamics mean they are laden with “low-hanging fruit” in terms of cost-effective efficiency and retroﬁtting opportunities.
This very interesting article inScience, “The MPG Illusion” by Richard P. Larrick and Jack B. Soll at the Fuqua School of Business in Duke University (Vol 320, June 20, 2008, p. 1593), points out the mathematically obvious truth that gas used per mile is inversely proportional to miles per gallon, which means that you have a steeper slope at lower MPG ratings, and diminishing returns at higher MPG ratings.
There are some important policy implications of this. Relatively small MPG improvements in the most gas-hungry vehicles pay off greater than larger improvements in already efficient car s (hence, it does make sense to offer tax breaks for modest improvements in SUVs versus tax breaks for hybrids, which typically replacing already gas-efficient sedans). Also, personal driving habits, especially for gas-hungry cars, can often times add or subtract a few MPG to a car’s efficiency on average. For example, a car that may get 25 MPG “average highway” will degrade to under 15 MPG if you gun it out of stoplights in city traffic. That’s a huge increase in gas consumed per distance driven, especially for the less efficient cars, whereas for more efficient cars it doesn’t hurt as much to goose the engine a bit.
Apparently the thinking that gas savings is linear with MPG is not uncommon. A survey of college students revealed that a majority of them shares this misconception.