The national average cost for gas is $2.67/gallon, Electricity $0.12/kwh. Based entirely on fuel costs, the Model S 85 costs $0.04/mile and the 20 mpg BMW M4 costs $0.13/mile. In order to save $200/month on fuel with the tesla, you'd have to drive 2200 miles a month. In the area I live, where gas is cheaper than average and electricity is almost twice as expensive, that number is 5000 miles/month.