I have a few highly held opinions about all this hybrid/electic stuff and they are like this
1st, the 1st and only real, try "hybrid" system cam from honda in the form of IMA (intergrated motor assist). Its the only system that constantly reginerates whether on the street, highway, or track. The prius is great... I guess, but if your on the highway, you are running off the motor all the time, and you are not charging anything back, because it only re-generates power under braking. However....
2nd, I have long held that this current "hybrid" B.S. is just that. Bullshit. Neither hondas IMA or Toyotas synergy drive are worth a damn. The only way to be truely hybrid is to use the gasoline motor to power feed power back into the batteries and let them power the electric motor. Its the best, cheapest, and smartest way.... However
3rd. Saying that a volt will get 253mpg is B.S. It wont, not ever, and thats just all there is to it.
I think that when it comes to vehicles like the volt, fuel mileage should be measured in distance per tank of fuel. Basically, if you leave your house in the morning with fully charged batteries, head off down the road. How far will the vehicle go before both its batteries and the fuel in the tank is gone.
So in the volts case, it wouldn't be 253 miles. It would be the average amount of miles that can be driven (say 40 for the battery only) and then add to that another 400 miles that the engine will operate to charge the batteries before it runs out of fuel... in this case the number would be 440miles. Now, run a couple of city and a couple of highway trips, average them out and that would be the number that needs to be given.
Other than that, they need to give a "drive cycle cost" amount. Say, how much is it going to cost to charge your batteries for the night, plus how much is it going to cost to fill up the tank from empty, Those are the only ways that I know of that the numbers would be comparable to anything else.