I am trying to take a speedometer out of a 1979 4x4 GMC truck and put it in 1994 GMC cab. Looking how to convert it. It goes from mechanical to electronic. If anybody has any ideas it would be helpful.

 

Not going to happen.....If you do manage to get a cable up there to the speedo, how you going to make it fit....It will be a butcher job and look like crap.

Go to the junkyard and get one for a 94 if you need one

 

Converting an already mechanical speedometer to electronic is futile... more work then what it's worth... not to mention it's calibration.
Converting transmission from a electronic speed sending unit to a mechanical is more do able, but I don't know of a late model tranni with a mechanical sending unit that might be adaptable. Then there is the issue of mounting this 1979 gauge cluster into a 1994 dash without it looking like a hack job.
I have to ask... Why are you doing this?