beltfed,
I'm guessing that there is enough iron in the coil cores to keep them operating in the linear portion of the (close your eyes non-techies) hysteresis curve.
The magnetic flux (read magnetic force) in the coil is dependent on the current and the number of wire turns. I'm presuming that the coils have differing numbers of turns to deal with the differing currents flowing in them.
The most accurate thing in my '49 was the tank sending unit. When I could hear it start to rattle, I was down to the last gallon or so!
