I'm certain that you would never run 85 amperes into a meter. The house I grew up in only had 60 ampere service. As I remember, this required #2 entrance cable from the meter. While the ammeter is effectively in series with the other resistances from the alternator, it wouldn't take the full 85 amperes. The ammeter movement was always put in parallel with a shunt resistance. I remember doing this in the physics lab. There are hand held ammeters that read the current flow inductively from the wire leading to the alternator. I'm certain that there is an inductive pick-up that sends a small voltage to the ammeter in the kits to put an ammeter in a car.
The ammeter was useful when automobiles had generators. Even then, many people didn't know what the gauge was for, so many auto manufacturers used a warning light to indicate that current was not flowing to the battery but away from the battery. The one time when I found the ammeter useful was when I put new brushes in the generator of my 1954 Buick and accidentally shorted the field coil of the generator to ground. This effectively let the generator run at maximum current. On cars with alternators, this wouldn't happen and I think that an ammeter with a modern car equipped with an alternator doesn't give much useful information.