Freightliner/allison Tranmission speedo problem

[skeeter2010]

Any truck mechanics have any idea why my speedo is does not read accurately all the time it typically reads 35 MPH when at 55 however does also read the correct speed about 30% of the time I suspect a bad prox probe someplace or a bad ground but not sure where to look in dash or at trans
You never know who is on the site figured it would be worth a shot

 

[vipermax10]

All depends what year truck and trans. if it is a 2000 or 3000 series and the trans shifts correctly with no codes the problem is more than likely a truck issue. If it is an old mechanical trans the trans doesn't care about the output speed sender, then it could be a loose tone ring, bad sender, damaged wiring or a bad speedo head. I haven't seen any of the electronic allisons use a second speed sender for the truck, but I haven't worked on many f/l's. More info is needed.