speed sensor

[fireballroberts]

My 'new ' tranny had a speed sensor with no guts. The 'good' speed sensor from my old tranny was put in. It only registers if the speed is above about 55 but then craps out again when the speed drops below 40. It worked great in the old tranny.
The old diff was 4.44 and the new one is the original 3.57 (?).
OK, so finally the question---are speed sensors calibrated to the gear ratio of the diff or is a speed sensor a speed sensor, a speed sensor?? Thanks