1991 blazer 4x4 auto trans and 4.3L

Speedo does not work all the time, but when it does the truck runs crappy. All other guages except the trip computer run fine.

 

The sensor on the tranny, the truck runs crappy cause once the speedo does work its getting a VSS signal and trying to relearn the correct way to run. Get the speedo fixed and I'll bet you get better gas mileage also.

 

Thanks for the response. I have thought about the VSS going out, but wouldn't it never work until replaced?
Also how do I determine if it is the sensor or the ECM?
Thanks,