90% vs 50 = 40% savings, . One gas one is 96.6% , many are 94 and 95%.
And a variable speed blower may save 75% on your old units blower KWH
cost. Not knowing how much it runs It cant be figured , but it could
easily save 15$ a month in electric costs and boost SEER for central
air by 1 seer. Plus not pullng inside air for combustion helps. Your
unit wasnt designed for efficency. Nothing was then.