MPG: Simply total miles driven divided by total gallons of gas burned. This is how the Volt reports MPG, although since the electricity is counted as "free" it doesn't give a true picture of the energy used. On the other hand, this is literally accurate. :)

MPGe: Counts all electric miles driven as 93, 94 or 98 "MPGe", depending on the model year of the vehicle. See this wikipedia page for the gory details, but in short, 33.7 kw*hrs of energy has been determined to be equal to one gallon of gasoline, and the Volt is rated at a combined 93 MPGe (0.36 kw*hrs/mi) by the EPA for the 2011 model, 94 MPGe for 2012, 98 MPGe for 2013 and up, and 106 MPGe for 2016 and up. This gives a better account of the true energy usage of the vehicle, in a number that's easily comparable to traditional cars. However, this is just an estimation - I cannot currently read raw kw*hrs from Onstar, so actual energy usage could be better or worse than this number. Hopefully we can get a real MPGe calculation in the near future. The formula is: Total Miles / (Gallons Burned + (EV Miles / EPA rating))

MPG_CS: Charge-Sustaining MPG, or what the Volt is getting on gas only.