Lately, there has been a lot of discussion about the 5 cycle MPGe test. Here's a link to more information. http://www.greenercars.org/guide_epameas.htm
As a reminder, the Model S currently rates at 89 MPGe which can possibly be improved further in the future by improving the charger efficiency (source: Q2 2012 Earnings Call).
The range rates at 320 mi in the EPA 2-cycle test and at 265 mi in the EPA 5-cycle test (85kWh battery).