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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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The Mustang test

Pollution from driven cars has fallen so fast it is now below that of parked cars in 1970

One small fact in my book has caught several readers' attention:

Today, a car emits less pollution travelling at full speed than a parked car did in 1970 from leaks.

My source for this remarkable statistic was Johan Norberg's 2006 book När människan skapade världen. In a translation he sent me it reads:

the average car today gives off one-tenth as much pollution per kilometre as in 1970. The fact is that a car travelling full speed causes less pollution than a parked car did in 1970, because of leakages.

Recently Henry Payne gave some more details, quoting an article in Autoweek -- which is interesting, but a little confused in one place:

The 1970 Ford Mustang pollutes more parked in a driveway than a 2010 Mustang does traveling down the road...

The operating 2010 Mustang is 98.5 percent cleaner than the 1970 with its engine shut off, according to Ed Kulick, an emissions regulatory planner in Ford's vehicle Environmental Engineering Department.

The '10 Mustang has demand-based fuel injection with no return lines, hydrocarbon impermeable fluorocarbon gaskets and evaporative emissions canisters that eliminate gasoline vapor seepage, even during refueling.

The '70 Mustang emitted the equivalent of 3.7 grams of hydrocarbon (HC) per mile sitting still, according to Kulick. The '10 is certified at 0.055 gram of HC per mile when cruising the interstate at 70 mph.

The 1970 'Stang had Detroit's first rudimentary apparatus to control exhaust emissions It met federal standards of 4.3 grams of HC, 39.6 grams of carbon monoxide (CO) and 4.1 grams of oxides of nitrogen (NOx) per mile. The 2010 generates no more than 0.055 gram HC, 2.1 grams of CO and 0.070 gram of NOx, for reductions of 98.7 percent, 94.7 percent and 98.3 percent, respectively.

The confusion is this: a parked car cannot emit pollution `per mile', so `the equivalent of' needs explaining. The key measure would be `per minute'.  Can anybody shed light on this? I cannot track down the original Autoweek article.