People are debating the efficiency standards for both electric and plug in vehicles. For traditional hybrid vehicles, MPG ratings make sense as the cars still require gasoline, use no electric power from outside sources, and cannot just use the electric engine for significant periods. The MPG numbers for hybrids are also still similar in scale to conventional cars. Toyota lists the Prius MPG as 51 city / 48 highway and a Highlander MPG as 20 city / 27 highway.
The MPG numbers for an all-electric vehicle have not made much sense and we need a new scale. Today, I could not find any MPG listed in either the owner’s manual or the Tesla web site. An old forum posting by a Tesla representative calculates the MPG according to the EPA mandate to be 256 MPG. The 256 MPG is based upon dividing the kWh/gal by kWh/mi = 82.049 / .321 = 256 MPG. The difficult part of this equation is a realistic number for the kWh/gal conversion factor. For the Roadster, the more interesting number is the simple miles / kwH. How many miles can I drive using how much electricity? If I invert the .321 kWh/mile, I can get 3.12 miles / kWh (MPKWH).
GM is now advertising the Volt, a plug-in hybrid, as 230 MPG, another meaningless figure. The number should be broken down into three figures: MPKWH when using the electric engine, a traditional MPG rating when using the gas engine, and a driving range for the electric engine. If these MPKWH figures were available for both cars, you could compare the efficiencies for two different cars for frequent short-range trips.
Treehugger.com has a discussion on this topic.