It'a a 1975 Lennox G12D2. It's really old and I'm getting it replaced. The unit shows an input BTU of 82K and an output BTU or named "bonnet capacity" of 65600. This makes not sense. I believe furnaces of that era had a AFUE of 60% or so. If you divide those numbers I'm getting 80% or it's outputing 80% of what's comming in?

Is it because the older furnaces calculated AFUE Differently?