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speedometer is slow with no changes

819 Views 5 Replies 2 Participants Last post by  to4tz
1991 - my speedometer has become slow on it's on with no changes made to the truck. It ranges from 12 to 15 mph slow depending on your speed. I have checked it with a Garmin GPS to verify. I replaced the instrument cluster with a working used unit 2 years ago and checked it at that time with GPS and it was spot on. Any ideas on what is going on?
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the speedo is either getting tired or the teeth on the speedo gear in the trans is getting worn and slowly slipping...

mine was real bad, i took it apart and it was disgusting, i cleaned it out completely and oiled the ship out of it... starting working perfectly....
did you take it apart and clean the speedo?? mine was totally stuck because of the old grease was solid rock... took the gears out cleaned em and put graphite on there and worked like a charm, if that doesn't work, get a new speedo.. tons of them in the junk yard.. gears as well...
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