Speedometer gone whacko!

[Dave Riggs]

The speedometer on my 1996 GMC Sierra 1500 (4.3L, auto, 2W dr, basic) has gone nuts! For a couple of days, I thought everyone else had gone crazy. I would be going 70 on a 55mph road and cars were passing me as though I were stopped. Finally (OK, I’m a little slow on the uptake sometimes), it occurred to me that instead of them all being fast maybe I was slow.
Yep. As near as I can tell the speedometer is close to accurate up to about 35mph then goes out of kilter. 45 reads about 60, 55 reads something over 70, 65 reads about 85.
This was a sudden occurrence with no predicate.
No work has been done to the transmission.
There was a starting problem that tracked down to a weak battery and a starter that checked good on a test stand but actually wasn’t. The speedometer problem began shortly after the starter and battery were replaced.
Thoughts?

Topic

[college man]

check engine light on or abs? With a scan tool, take the vehicle for a ride and see if the MPH is read. If not seen then vss
would be suspect. check the vss connection at the rear tail shaft of the trans.

[Author]

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