I have a problem where the speed is running 5 miles an hour to fast I have 323 posi rear and a th400 tranny.Had one of my freinds drive a long side of me and found out it was running to high.the car originally had a 350 engine know has a 455ci motor but not sure of the origanal tranny the odometer reads 27,250 which I think had originally 127,250 miles on it

 

could have a different speedometer gear or driven gear in the trans now for the rear end that is in there. You need to find out the code on the rear end and then check where the speedometer cable connects to the trany - there should be some numbers on the shaft like 40,41,42 that will tell you what spedo gear the transmission is set up for.

 

you can call the highway patrol and ask who calibrates speedo's ...

 

If the car originally had a 350, then it also originally had a TH350, which means that the TH400 was not original and almost certainly does not have the correct speedo gears in it. Change them and the problem goes away.