The only accurate way to do this is to install a KWH meter either in the car's charger or in the single location where you charge the cad and then measure actual power consumed. Multiply power consumed by your local rates and then convert that in gallons via price per gallon. to get an mpg. Or simply calculate your costs per mile from the power/price numbers and miles driven. Do any of these electric cars include a actual KHW meter in their charging system?