The MPG myth

In terms of vehicles' carbon emissions, a little goes a long way. A couple of years ago, Ford Motor claimed that just a small improvement in fuel efficiency in all cars would go further to reducing gas consumption and carbon emissions than large contributions from single models. New research suggests the automaker was spot on.

A new study published in the weekly Science journal argues that fuel efficiency is misunderstood, as people falsely believe that the amount of gas guzzled by a car decreases in proportion to the vehicle's miles per gallon (MPG).  In actual fact, the relationship is curvilinear, meaning that seemingly tiny improvements in an engine's MPG yield massive improvements in terms of fuel efficiency. The authors of the study argue that linear reasoning about MPG leads us to undervalue small improvements on inefficient vehicles, and that their discoveries should have implications for both public policy and research on environmental decision-making.

MPG fallacy

It is a case of shuffling the formula that dominates fuel economy. The authors believe fuel efficiency becomes more transparent when expressed as gallons per mile (GPM) rather than MPG, or the amount of gas consumed for a given distance. This bears elaboration: to cover 10,000 miles, a car that gets 12 MPG consumes 833 gallons of gas (10,000/12); a car that gets 14 MPG consumes 714 gallons (10,000/14). The roughly 120-gallon reduction in fuel used is larger than the reduction achieved by replacing a car that gets 28 MPG with a car that gets 40 MPG over that distance.

The table below illustrates that perceptions of improvement vary greatly with the actual reduction in gas consumption.

The benefits of improving gas mileage
Change in vehicle pairs (MPG)	Perceived ranking as savings (mean)	Actual ranking as savings	Actual reduction in gas consumption per 10,000 miles
Source: Science
34 to 50	1.18	3	94.1
18 to 28	1.95	1	198.4
42 to 48	3.29	5	29.8
16 to 20	3.73	2	125.0
22 to 24	4.86	4	37.9

All this suggests it is worth the extra expense -- for the sake of the environment -- to improve the fuel efficiency of notorious gas-guzzlers such as Sports Utility Vehicles (SUVs) and high-performance cars, if only by a few miles per gallon. Tax breaks on 'green' SUVs that have an MPG performance just slightly superior to sister models still look like a good idea.  GPM also makes cost savings from reduced gas consumption easier to calculate.

Likely industry response

It remains to be seen how the motor industry will respond. The European motor industry has signed a voluntary agreement with the EU to reduce carbon dioxide emissions, and forcing its car fleet from an average 35 MPG now to 49 MPG by this year, and 57 MPG by 2012. Manufacturers may concentrate on squeezing a few more miles per gallon out of existing strong performers to bump up the average MPG of its fleet, rather than investing in new engines to improve gas-guzzlers.

Investment should not just be ploughed into new engines, which now have to be renewed almost as often as the fashion-driven changes to car bodies -- about once every seven years. There is now general agreement amongst automotive engineers that there is considerable scope for energy saving in the design of more efficient transmissions. That means yet another call for capital intensity and the creation of new plants and production equipment for redesigned gearboxes that in the future will follow some of the principles of Formula One transmissions technology.

None of this solves a policy problem: the lack of a common measurement for people to compare the consequences of their activities across a range of daily actions. As the authors of the Science report write, choosing a more efficient car is just one means to reduce greenhouse gas emissions. "Arming consumers with information about the relative greenhouse gas emissions [allows them] to make beneficial trade-offs in their daily decisions."