Reversive cluster mileage

I bought my 87 5.0 Mustang new. It had an 85 mph speedometer. It was common then to replace it with a Ford Motorsports 160 mph speedo.
I took the new one apart before I installed it and set it to the mileage that the original one had on it. I don't recall it being any trouble. I still have the car and the paperwork (and the original speedo). As I remember, the new one came with a sticker to put in the door jamb to denote the changed speedo.
Of course, I didn't need it. I'm sure it will befuddle whoever buys it when I'm gone. But the paperwork in the file that will go with the car will prove everything out.
But I think by then, who'll care? But they will know that the indicated mileage will be accurate.
Also, how many cars have had undocumented gear ratio changes (and the lack of accurate speedo documentation) or just have disconnected speedos? Mechanical speedos can be easily changed. Even today, people don't rely much on what an odometer reads when they buy a car.