1. Calculate the number of Joules from battery storage it takes for the car to go the required distance at the required speed.
2. Using the value from step 1, calculate the total number of Joules (allowing for loss in transmission, charging, etc.) it requires from a power plant to charge that battery with those Joules from step 1.
3. Divide the result from step 2 by 1.3x 10^8 (the Joules per gallon of gasoline) to get the value in gallons of gasoline.
Presto! You're now comparing apples and apples.
I'll bet a dozen donuts that you will NOT get “230 MPG.” In fact, I suspect it will make most compact cars look pretty good.
If your vehicle is a plug-in hybrid, you can use the result from step 3 along the efficiency of the hybrid's IC power plant.
Of course, since this is allegedly about the environment, you could calculate the mass of “pollutants” produced at the power plant to create those Joules at step 3, vs. the “pollutants” produced by a comparable automobile for the same speed and distance. It might make the electric car look better. Maybe.
And, of course, we're not including the toxic products from building and disposing of the battery. We might need to add that our electric car's wastes.
Anybody see any holes in this?
Yes, coal is cheap and domestic, gasoline is expensive and imported.