I already get much better than that in the city. I just used those numbers because they are the only reference I have. My 90 day average is about 25 which is about 60% hilly/city and 40% highway. 6.6% better would be 26.6 mpg. If I assume no benefit on the highway the the advantage would be about 4% which would return 26mpg average.
I'm hoping to get a good idea before the winter gas starts
