My speedometer is acting like a voltmeter or ohmeter or something. Every time any electrical device is activated the needle wags. Windows, locks even the windshield wipers. It becomes very annoying and distracting. I figure it must be a loose connection because road bumps affect it too but where should I start. Also the speedometer is WAY OFF. Ever since I bought the new speed sender. The truck is auto with 3.55's what one should I look for? I thought the guy at the parts place could handle it, obviously not. The speedo is off by probably 10-15 and I know it will eventually get me into trouble! Thanks for the help.

 

Sounds like you have a bad ground some where if other electrial devices affect it. As for the speed sensor, did you get one for a gasser that came with 215 75 tires?

 

I vote for a grounding problem.
The sensor is the same for all of em & they change the gear in trans &/or the little square gear box between sensor & trans to get proper calibration.