I have a 400i, with a swinging fuel gauge. With precisely a tank 3/4 full, I've got the following readings on the gauge : 1) 3/4 (Ignition off), 2) below 3/4 (750rpm) 3) full (above 2000rpm). That's a Veglia from 1982, presumably found on many Ferrari of that era. Is there any voltage regulator built into these units?
The Veglia analog fuel gauge has internal compensation for changes in battery voltage. See the attached photo. I suggest you remove the gauge and do a bench test using a fixed resistance and varying the positive voltage. The pointer might move when you brake or accelerate as the fuel shifts within the tank.
