They bridge the gap between conventional hybrids and full electric vehicles.

Essentially the automotive equivalent of the combination VHS/DVD players that helped ease the transition from analog to digital home video, plug-in hybrid cars bridge the gap between purely petrol-powered and full-electric vehicles.

Plug-in hybrid vehicles (PHEVs) come with a larger battery than conventional gas/electric-powered models that allow them to run for an extended number of miles purely on battery power to bolster their fuel economy and reduce tailpipe emissions. While their all-electric range on a charge is shorter than full EVs, PHEVs effectively eliminate so-called “range anxiety.” Once the battery becomes depleted to a certain level, a PHEV operates like a conventional hybrid, with a subsequent operating range that’s limited only by the amount of gas in the tank.

A PHEV’s full-electric range can be little more than a short sprint on some high-powered luxury plug-ins – it’s just 14 miles in the Porsche Cayenne S e-hybrid – but can run as high as 53 miles with the Chevrolet Volt and 97 miles with the BMW i3 REX; the latter use small gasoline engines as “range extenders” to generate electricity that powers their electric motors once the batteries become drained.

As one might expect, plug-in hybrids cost more than their non-corded counterparts. The Toyota Prius Prime, for example, caries a sticker price that’s around $3,600 higher than a base non-plug-in Prius; in the case of the aforementioned BMW i3, the plug-in hybrid is priced $3,850 higher than the full electric version. Unlike standard hybrids, however, plug-ins are eligible for a one-time federal income tax credits that range from $3,500 to $7,500, based on the capacity of the battery used to power the vehicle. If your main concern is the financial bottom line rather than of the environmental variety, you’ll have to run the numbers to determine if a PHEV will be cost-effective based on current gas prices.

We’re featuring the top 10 most fuel-efficient plug-in hybrids in the accompanying slideshow, with both combined city/highway gas and electric equivalent (mpg-e) ratings coming from the EPA’s fueleconomy.gov website; we’ve ranked them according to their fuel economy and EV ranges. All prices cited are for base models and include the automaker’s destination charge, but not options, taxes, or license fees; cash rebates or other sales incentives may be in effect for some models.

When fuel economy matters: