Thats because the 3 has a 2.3 4cyl and the Civic has a 1.8 4cyl of course there is gonna be a difference but that 2.3 is the same motor I had in my 6 and gas mileage went down the pooper for it after it hit like 60k miles and it was only getting like 23/24 mpg on the highway and most of the tune up parts you can only get from the dealer and they are $$$
Civic gets close to 40 mpg and the auto gets close to 42 mpg depending on how you drive too lol :burnout: