When I do a lot of highway driving, say an hour on 95 down to daytona and back, I can do 24 for the whole tank which includes a few trips to the office (18 miles away) where it's a mix of stop and go and 70-80.
It's the stop and go and very short trips that really craters the mileage. On a longer 3 hour trip to Tampa I can do 26.
It sounds like the M2 is about in the middle of where you want to be.....so do the M2