The fourth chart below shows a measure of vehicle efficiency from 1975-2007. Plotted on the left-axis is what is called “tonnage mileage per gallon.” Simply stated, it is the “composite” mileage per gallon of all light-duty vehicles multipled by its inertia weight in tons. In other words, a high ton-mpg indicates a vehicle’s improving ability to move weight (how many miles a one-ton vehicle can travel using one gallon of gas). You might consider this a measure of a vehicle’s efficiency – and some indication of how much an improvement in fuel economy car manufacturers can currently produce by sacrificing engine performance. Just as an increase in a car’s fuel efficiency at a constant weight is considered a move toward “efficiency,” so too is an increase in how much weight a vehicle can carry, while still getting the same fuel economy.

I’ve never needed engine performance myself, so would welcome a roll-back in performance – particularly since I drive like an impaired old-lady. I am, however, a little surprised that some of these efficiency gains had not been translated into lower-performing, higher fuel-economy cars, over the past 30 years. Was the demand for fuel economy really that low? One reason I did not buy a Saturn after I sold my first one (an SL2, I loved it) was that the fuel economy of the newer versions of that car (around 2001) were considerably worse than the first one I bought in 1997.

In any event, light-vehicles seem to have realized a nearly 70 percent increase in efficiency over this time period. So, by my thinking, if a typical small car got 25 miles per gallon in 1975, then if all of the efficiency increases went toward improving fuel economy, so that a 2007 car that performed similarly were available for purchase, that typical car would have an mpg rating today of over 42mpg.

I’ll remind readers that sacrificing performance for fuel economy will certainly lead to much more dangerous driving conditions if that economy is obtained by making vehicles lighter. What you “save” in fuel mileage might be more than offset by the increased (in expected value terms) risk you face when driving. What the data here indicate to me, however, is that fuel economy could have been improved without sacrificing safety. In consideration of my other query – why consumers can’t make such choices, I think it is largely explainable by the industrial organization of the automobile industry.

The data come from the EPA. Enjoy the charts!

Chart 1

Chart 2

Chart 3

Chart 4 – Efficiency Gains

Begin with the current inventory of carbon dioxide emissions – CO2 being the principal greenhouse gas generated almost entirely by energy use. According to the Department of Energy’s most recent data on greenhouse gas emissions, in 2006 the U.S. emitted 5.8 billion metric tons of carbon dioxide, or just under 20 tons per capita. An 80% reduction in these emissions from 1990 levels means that the U.S. cannot emit more than about one billion metric tons of CO2 in 2050.

Were man-made carbon dioxide emissions in this country ever that low? The answer is probably yes – from historical energy data it is possible to estimate that the U.S. last emitted one billion metric tons around 1910. But in 1910, the U.S. had 92 million people, and per capita income, in current dollars, was about $6,000.

By the year 2050, the Census Bureau projects that our population will be around 420 million. This means per capita emissions will have to fall to about 2.5 tons in order to meet the goal of 80% reduction.

It is likely that U.S. per capita emissions were never that low – even back in colonial days when the only fuel we burned was wood. The only nations in the world today that emit at this low level are all poor developing nations, such as Belize, Mauritius, Jordan, Haiti and Somalia.

Read the whole thing in Monday’s Wall Street Journal.