Vehicle: 1987 WS6 Trans Am
Dash: Digital
Engine: 305 TPI

Allright, this is driving me nuts. The Tac is almost completely dead, it hovers around 3 points: 0, redline, 2200 rpm. There doesn't seem to be any reason or pattern to which of the three it is at for any given time. Now, the speedo is off by approx 15 MPH. It seems to be halfway linear, though it does go to zero when stopped, and sems fairly accurate under 30.
Then I tried clocking the mileage. Its racking up an extra 6-7 miles for every 40 miles driven. What the heck is going on here? Where should I start? The rear end/tires/etc are all stock. Can the TAC being out cause these other issues?

Vehicle is a 5 speed, if that helps at all.

 

not for sure but my experience with digital dashes never had one in third gen but had a s10 blazer with one and it started doing sort of the same thing then a little while later the speedo and tac would just go blank but everything else would stay on and work fine oil pressure etc. I ended up buying a different dash and all the problems went away. not saying it is your problem but something to look into because digital dashes have there own computer board and after 20 some years that stuff does tend to go bad and digital dashes while cool people have problems with them. I remember people having to get them replace when they were newer cars.