The author of the piece (Eoin O'Carroll) worked out, with help, the likely effect.
How big is the actual effect? I asked Jim Kliesch, a clean-vehicle expert with the Union of Concerned Scientists, who has reviewed a number of studies on the subject. He walked me through the math, and we calculated that doubling fuel economy would increase driving for the average person by a little over 7 percent.Thereby he believes that Jevons Paradox does not apply to cars and fuel efficiency.
“It is certainly not of the magnitude to have any significant impact on energy savings,” he said.
I think he misunderstood the Paradox, so this is what I wrote in a comment at the site.
I believe you misunderstand the application of Jevons Paradox. In his original example, Jevons pointed out (as you note) that increasing the efficiency of a locomotive led to an increase in coal demand for coal, their fuel supply.
But you have considered that increasing efficiency would increase the mileage driven by an individual vehicle. That is not what he said, nor, realistically what will happen. Rather it is that by lowering the cost of locomotives, people used more of them, and thus demand rose, not that an individual locomotive drove more miles.
Relevant to the current debate the example is the Tata Nano - by improving manufacturing efficiency to create a car cheap enough for a lower stratum of Indian society (and later the world) their ability to afford the car will drive up gas consumption.
There are many examples - computers, i-pods, cell phones etc etc. It is in the marrying of price with improved efficiency that provides the engine for the Paradox.
And yes, in its correct mode of use, I expect Jevons Paradox to continue working.