HYBRID cars made immediate headlines nearly a decade ago, when Honda and then Toyota introduced models that offered drivers moon-shot improvements in fuel economy and exhaust emissions.

Today, those leaps of progress are proving tough to repeat: as new technologies develop and gasoline-electric powertrains become available in a wider range of models, the gains over existing hybrids seem more incremental than startling.

And even when new twists in technology do arrive — developments that include plug-in hybrids, which can be recharged on household current to give them more driving distance on batteries alone — it may be impossible to give buyers a measure of how much the advances help because there is no test to measure their mileage.

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http://www.nytimes.com/2007/11/04/automobiles/04PLUG.html