I have the above mentioned truck. It came with the oversized tires, shifted rough and a speedo that floated at 35 through 45 mph and beyond. I switched out the 18 tooth speedo tail gear for a 16, this eased the shifting but it caused the speedo to lay down until 25 (est.) and moves slowly jumping its way up. It never reaches the real speed of the truck's moving speed, and the downshift seems to be really quick downshifting out of overdrive. I changed out the speed sensor on the transmission. This didn't hurt, but it didn't help either. It runs well through the gears, except for the down shifting out of overdrive. Does the speedo control the shifting in some manner or is the problem in the belly of the beast's transmission>

 

In 1991, the speedometer was mechanical and has nothing to do with the tranny. You might have a bad speedo cable or the speedometer itself isn't working right. If you pull the gauge cluster, you can turn the speedo with a drill and you should be able to figure out which is bad.

As for the tranny not shifting right, can you explain more about that? I'm not quite sure I follow.

 

the transmission shifts smoothly and correctly 1, 2, 3, overdrive on a flat / level road. When I apply any sort of acceleration/load the transmission will drop out of overdrive. It seems a little touchy or quick to react. The shift is passive yet I can see the tach jump. When I ease up it drops back into overdrive. It is the same with a grade. Is there a "drop down/ kick out" switch for the overdrive.

 

It could be that it's shifting correctly, and that when you're driving on flat ground, it's almost at the shift point. A little extra throttle and it thinks it needs a lower gear. A lifted truck with big tires and a stock gear ratio will do that. What gears are in the truck?

 

well, it's a stock 3.08 gear. A little time, and maybe I'll get use to the shifting. I thought that there might be a setting or chip somewhere that needed to be changed out because of the tires. thanks for your help. I'll pull the speedor and look it over tomorrow. Question. Doesn't the speed sensor work off the movement of the cable

 

I would put the old 18 tooth gear back in since you changed the VSS on the trans. It could have been the trouble with the shifting all along. Replace your speedo cable.

 

Thanks, I'm going to pull the cluster, inspect the speedo head, and replace the cable. My understanding is that the 16 tooth is necessary to compensate for the larger tire size and a proper speedo reading ? Thanks again for the input.