When I first read this thread i thought i had really bad mileage. I was getting 300 or even 280 with a tank. But i realized that an oil change was due and i drive a lot in the city. I seafoamed last week with an oil change and this weekend i went down to atlantic city. I got approximately 260 miles with half of my tank (i factored in the fact that the fuel guage goes over full) so i would say i can get 500 miles per tank give or take 20 with all highway.
I have no idea whether or not the seafoam helped in my mpg or not but ill see during this week and compare it to my 300 per tank.