There is only ONE way to know. You need a professional Manual J calculation done on your home. It might cost a few hundred or maybe even free, but you get what you pay for in many cases.

this is how you find out how many btu's you need. The 75,000 that is on the existing furn is for the imput and the output would be 90000. The way you should find btu's is take your sq. ft. 1600 and multiply it by 40. 1600x40=64,000btu's. The formula is reversed for finding the taonage of your A/c. Take 1600/40=4 ton.

That is funny!

Not only does crob manage to make a 75,000 input furnace and produce 90,000, it that a 125% efficient furnace?

His sizing suggestion is exactly how people get caught oversizing. 4 tons of AC in a 1,600 sq ft house in Detroit?

Too start with the exiting is 2.5 and was probably sufficient, please don't take any short cuts and get a competent appraisal with a detailed heat loss.

crob's formula for sizing a new furnace assumes 2 things, 1) the original furnace was sized correctly and 2) the heat loss has not changed at all. New doors, windows, insulation, weatherstripping, etc will lower the heat loss, which will reduce the size needed for the replacement furnace. His figures for in/out are reversed.