check this thread out.
http://67-72chevytrucks.com/vboard/s...ighlight=4l80e
This is something you will need to check inside the tune if you can or have your tuner check it. These transmissions are electronically controlled so they must have the correct speed input.
As noted in my other thread my speedo worked correctly however the one I circled is what the trans uses.
look these over if that is not it then I will look more.
There is also "limp" mode but I would check here first. Mine was doing the same thing however when it shifted so early my TC was loose enough it almost wouldnt pull itself.
Dave