I have two M3's and mine on the interstate get 27 for the M3/4 and 25 for the M3 convertible.
Mixed 24-25 for the M3/4 and 23 for the M3 vert.
Never produced anything over 28 mpg and seldom see 28 mpg but consistent 27 mpg.
I have owned my M3 since I bought it CPO from the dealer in 2000.
Hope that helps...