Get your oil or gas burner tuned every year by a qualified technician. The efficiency numbers do not apply to burners that are out of whack. Ten percent here and there adds up.
If your gas furnace exhausts via a smoke pipe into a chimney, you are losing money.
The modern high efficiency gas furnace vents directly to the outside. It burns gas so well, that there is little heat left and can be vented with plastic pipe
If you have the chimney type, you are losing at least ten percent of your fuel up that chimney.