If you have had 16 " on all the time and your calculations are correct. Then your fuel mileage may be off a bit After I had mine in a speedo shop here in Portland 😆 My mileage went up////?
Fast forward to yours. I would take it to a good shop and they can A) calibrate it or B) put in an adapter on the tranny . Mine cost abot 150 US
They put it on a roller system and did the gear change in the adapter.