My car is using 400 w/mile out of the wall. EPA fuel consumption rated it at 19-21 mpg. Figuring an average of 20 mpg for my car with an ice. That work out to 20*400 or, 8000W per gal of gas, approximately. I don’t know where I heard it. I am not smart enough to figure the math out but it worked out pretty close for my car. It will not be an exact number for every situation but it seems to be pretty close.