I recently bought a 2010 Honda Civic LX 1800cc and I've noticed the gas mileage has been terrible, a full tank doesn't even last 215 miles (30% has been highway driving), I calculated the mpg and it's around 16-17. My mechanic checked and it has no trouble codes and everything is in generally good shape, the car only has 39K miles so it shouldn't be such a gas hog. The air filter isn't that dirty and the spark plugs are in good shape, the tires are a bit worn but I doubt that's what's causing the bad mpg, the motor oil and transmission fluid were changed recently. What could be the problems from the most simple things to more complex issues? Thanks!

(I'd like to add it has an automatic tranny and I bought it from a private individual, no salvage title)

 

Did you ever figure this out? Just curious.