why does my speedometer jump when i do 50mph after replacing the whole rearend on 1990 Ford F-150

changed out the whole rear end and now the speedomoter jumps when i get to 50mph.It jumps to 100 then back down etc.

by in West Monroe, NY on December 01, 2012
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2 answers
ANSWER by , March 02, 2015
check the differential to see if you have a speed sensor near the left rear wheel which might not be working correctly with the computer . The one you put in might be bad.
ANSWER by , December 02, 2012