I have an even stranger problem than most. I pulled the engine and the ECM out of my 1991 TBI, 5-speed and stuck in a warmed over 350, carbureted. My speedometer works, but not after 75MPH. That is as fast as it will register no matter how fast I push the car. This is a 140 MPH gauge cluster swap. Any ideas why it will read ok until 75,and will go no higher, or should I be glad that my speedo is working at all given there is no computer.