a combonation of both, MAYBE. but going from 17 to +2/4 MPG over stock is a stretch for just an air filter change. Also the ratings are usually with a stiff tail wind and over inflated hard ass compound narrow tires to squeek everything they can out of them for numbers sake!
it takes alot to make a big difference in mileage. rolling resistance, rotational mass, wind resistance, dt loss, weight, etc all play a bigger part when a motor is running good to start… performanc mods dont really do much unless they are drastic changes.