depends on the driving conditions and temperature, normally I get around 16.5mpg on the highway and 14ish around town, now that it is getting down into the teens here in the northeast I am getting about a mpg or two less. it's just the price of admission, it takes a lot of gas to go slowly but get there in style. p1178 is like that phantom unwanted relative that randomly just shows up out of the blue, my experience with it leads me to believe that it's vacuum related