What kind of miles per gallon will you see on the EPA stickers of plug-in hybrids as they come out in the next few years? The miles per gallons for plug-in hybrids will be rated lower than traditional hybrids! Why? Unlike traditional hybrids, to get the added miles per gallon from a plug-in hybrid, you have to do something: plug it in. If you don't, the un-used part of the battery is just dead weight and that means lower miles per gallon.
Plug-in hybrids will probably leave part of the battery flat waiting for you to charge it. The car's software could fully charge the battery using regenerative braking but why would a driver go through the effort of plugging in the car if the battery meter was always topped off? A solution to this problem is an "I will not plug you in" button that tells the car to fully charge the battery and don't wait for the driver to plug it in.
I don't think the major automakers will go for a software solution however. I think the first plug-in hybrids will just be better hybrids and get better mileage than traditional hybrids only because they have larger batteries. This will be regrettable because it means plug-in hybrids will hobble the development of traditional hybrids. Plug-in hybrids will only be the panacea we hope for if the auto makers, EPA and drivers get it right.
We want to know what you think. If you were the EPA, what would you put on the sticker for a plug-in hybrid? Would you put the highest possible mileage, the lowest or an average based on plug-in hybrid use studies? If you were the car maker, how would you weigh the options? Would you make every hybrid you made a plug-in or market it as a high end option? As a consumer, what's the minimum miles per gallons increase would convince you to take the time to plug in a car?