When i do freeway driving yes i do use cruise control most of the time.But city,not really.I get decent mileage. Can tell you this.I get better than i did stock.I don't know the exact numbers,but my gas does last me allot longer than it did when i had my stock tires and no tuner.I figure i get 16-18 city.I will have to track my gas usage sometime. You want to save on some gas.Turn your fuel dump down,lower your idle.I did this and thats when i started seeing better mpg's.Even tho i turned it down,i still dont have any throttle lag which is nice.
You going to be off by 2-3 mph.But just to be on the safe side.Take your GPS with ya to see.You dont need a speeding ticket