Under steady-state cruising the computer in your car is controlling the throttle body (among other things) to regulate the amount of air ingested by the engine. Remember, the throttle is an intentional intake restriction. A decrease of restriction in the intake tract elsewhere will be matched by a slight closing of the throttle body to maintain the level of airflow the engine needs for the same steady-state cruise. The pumping restriction seen by the pistons will be the same. They don't know whether it is a clogged filter, or the throttle body causing the restriction. Your fuel economy will be the same. Full-throttle acceleration will be improved with a clean air filter.