Hi Rod , I do not get nearly as many miles out of a tank as you. My 400 miles on a tank was for a rather steady 75 mph for more than 5 hours. As with you my city driving drops drastically, I would say generally in the 16- 17.5 mpg range. I have a MTM- Hoppen chip which I'm very happy with, but I think it makes the care a a little thirsty. Regards, Shawn