The government rates the Model S at 89 MPGe. As gregincal pointed out to me, this is based on the energy content of gasoline (see his post #6 below). However, I think it is useful to calculate another type of [cost-adjusted] MPGe which incorporates the local cost of electricity and gasoline, given the widely varying electricity costs--and also gasoline costs--around the country. This allows a cost comparison to one's ICE cars. So, I calculated my cost-adjusted MPGe, considering that I pay 5.79 cents per kWh, and my cost for gasoline is $3.99 per gallon (I used the cost for premium, since that's what all of my cars take). I'm helped here by the relatively low electricity and relatively high gasoline costs in Chicago, and also my use of an alternative electricity supplier. For me, that gives an MPGe of 181*. I made a simple spreadsheet to calculate this, which is here. I used the formula MPGe = (miles per kWh)*(cost of gasoline per gallon)/(cost of electricity per kWh). [Miles per kWh is 100/(kWh/100 miles)]. The Model S with the 85kWh battery pack is rated at 38 kWh per 100 miles. So, what is your cost-adjusted MPGe? *I posted this later on, but adding in all of the transmission fees, distribution fees, and taxes on my electric bill brings this down to a less-lofty 114 MPGe. Still not bad, though.