Reduce the load before chasing a rating
AFUE is a standardized measure of how much of a furnace's fuel energy is converted into useful heat over a representative operating cycle or season. It is useful for comparing equipment, but it does not determine the home's total gas bill by itself.
The building's heat loss determines how much useful heat must be delivered. Weather, thermostat settings, air leakage and insulation can therefore dominate seasonal consumption.
Gas rates and fixed utility charges also affect the dollar bill even when fuel usage remains similar.