All cars should be rated not by miles per gallon but grams of CO2 per mile.

The average American ICE car generates about 400 grams of CO2 per mile in just fuel useage. But also, electric cars also have a CO2 per mile rating. This depends on where the car is, which power grid they are on and how that generates power, but this is a known quantity and should be reported to the consumer. The car knows where it is, the companies could very easily link the power grid info to the car. On top of the carbon per mile, each car should know the expected product lifetime as well as the amount of carbon required to make the thing. Amortize the carbon cost of production over each mile for the expected life of the car. If I buy an electric car that took 23 tons of carbon to make and I get my electricity from coal power plants, I should know at time of purchase that my car generates 800 grams of CO2 per mile.