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.
You mean 400 Wh, Watt-hours, not Watt. Watt-hours is a unit of energy, Watt is a unit of power, which is energy per unit time. It takes energy per unit time, power, to keep a car moving at constant speed, so it consumes energy per time and energy per mile.
Most of the energy stored in the gasoline is dissipated as heat in an internal combustion engine, with only roughly 20% used to move the vehicle. You are neglecting that 80% in your calculation. So 8kWh is only about 20% of the energy used from the gasoline, and 8kWh/0.2 = 40kWh, 32kWh dissipated as heat and 8kWh used to move the vehicle, which overestimates the energy in a gallon of gasoline a bit. If we assume all ice's are 20% efficient, your calculation certainly gives some guidance, but I think efficiency actually varies significantly, especially from older to newer vehicles. My car was rated at 32 mpg by EPA, and uses about 216 Wh/mile from the wall. The product gives 6912.
Also, if you have an electric heater in your ev, using it will increase your energy/mile, but using the heater in an ice-powered car will not effect your mpg as the heat would be dissipated by the engine whether you make use of it or not.
I think there is less variation using the EPA number for Wh per gallon of gas, but that is just an average since it varies a bit with grade of gasoline.