I have a 1993 F150 that has a problem with the speedometer needle acting erratic. Once you get over about 20 mph. the needle on the speedometer begins to bounce between your true speed and 5-10 mph under. I changed the vehicle speed sensor at the rear differential. The problem still exists. Is a bad ground to blame. Either at the speedometer or the speed sensor. Would this problem effect the engine's management system also. I seem to be getting worse mpg. and pinging on hills.

 

Yes it might affect the engine management system. I would follow the wiring from the sensor and check it for corrosion and bad connections. Also, is the mechanical condition of the rearend ok? That could affect the ability of the sensor to read.