In the near to intermediate term, hybrid vehicles can only reduce fossil fuel consumption by a few percent. Tey just can't be built and added to the fleet fast enough. Ditto for PHEV, and here the potential "battery bottleneck" seems even more important.

Ethanol fuel for ICEs and biodiesels for diesel engines another few percent, with early limitations in deployment and later with the total quantities of alt-fuels available.

Improving the fuel efficiency of the current vehicle fleet could be more rapid, with just as large an effect as the above. But these are ideas we have all heard before and boy do they bore the public at large: Correctly inflate tires, drive not above highway speed limits, new air filter, tune-up, combine short trips. I'm sure you've heard them all.

Photovoltaic and wind energy generation can be another few percent, assuming the investment/incentives are in place.

Energy conservation in homes and industry has actually been a minor success story, but there is plenty more to do. Good for at least a few more percent.

My point is, to lick this thing, we need them *all*. Togetherm they might be enough.

You're probably right... Hybrid->PHEV->Electric will take a LONG time, and massive battery tech upgrades (although I'm very hopeful, since all mobile technology relies on the metallurgical wizardy of batteries).

HOWEVER, I'd like to proudly proclaim that there is another reason you might want to drive a Prius.. and this is a significant improvement over a non-AT-PZEV car: reduced emissions. I honestly like the fact that when I'm in "stealth" or "golf-cart" mode, I'm almost emitting no greenhouse gases (tire particulate is a different story, but isn't as globally bad as the gasses).

So yeah, you're probably right, but I still <3 my Prius

time is of the essence. the average new car today will be on the road for about 17 years. that means, IF 100% of vehicles sold today are PHEV vehicles, it will take nearly a decade to see any major reduction in consumption.

Dave,

Approx 17 million cars/trucks are sold annually in the US, and the total number of vehicles on the road is fairly constant, so that means that annual sales/total fleet is the annual turnover ratio -- quite a bit higher than you suggest.

Also, if the really old cars tend to be reserve cars in the family; or if the gas guzzler took on that role, then the total effect on reducing fossil fuel consumption by introducing highly efficient plug-ins would be magnified beyond there numerical ratio.

I'm pretty sure my family is not typical, but we have a 25 mpg Subaru in addition to the Prius, yet the Prius accounts for close to 90% of our driving.

I don't have a good number for the US fleet. Anyone ?

i find it hard to believe that the number of cars actually being used on american roads has been static for any extended period of time

I think it parallels the population. Not static, but growing slow enough that for purposes of this discussion over a 5 year period or so, is close .