29-19=10
10/2=5
19+5=24 avg.

If you are getting anywhere near 24 then consider yourself lucky.

I have friends who drive Lexus IS300s and they get only 18/25 yet have a smaller engine and a lighter body.

Keep in mind the 24mpg can only be achieved if you use a very light foot. Anything higher would mean that you are constantly driving in 5th gear travelling no faster than the fixed cruising speed of 70 mph (the faster you go the more friction due to drag).