Speedometer not working properly

Good morning,

I would suspect the governor could be the issue.

It is located at the back of the transmission and is run by a output gear. That could be the shifting going into overdrive.

As far as the speedometer, did you replace the sensor on the tail shaft?

The vehicle speed sensor (VSS) sends a pulsating voltage signal to the ECM, which the ECM converts to MPH. This sensor mainly controls the operation of the transmission Torque Converter Clutch (TCC), shift light and cruise control (if equipped).

Since there is more than one possibility for transmissions, I attached the possibilities for both. Your transmission may not have a modulator valve.

https://www.2carpros.com/articles/automatic-transmission-problems

Roy

Description
The engine vacuum signal is provided by the vacuum modulator which consists of an evacuated bellows, a diaphragm and a spring. These are so arranged that the bellows and spring apply force that acts on the modulator valve so that it increases modulator pressure. Engine vacuum and a spring oppose the bellows and spring to control modulator pressure.

To reduce the effect of altitude on shift points, the effective area of the diaphragm is different than that of the bellows. Atmospheric pressure acts on the resulting differential area to reduce modulator pressure.

Operation
A vacuum modulator is used to sense engine torque input to the transmission automatically. The vacuum modulator transmits this signal to the pressure regulator, which controls line pressure, so that all torque requirements of the transmission are met and proper shift spacing is obtained at all throttle openings.

Vacuum Modulator Valve
Provides modulator pressure that senses engine torque and vehicle speed. The vacuum modulator speed is used to vary the shift points according to throttle opening and to raise line pressure proportional to engine torque.

2-3 Modulator Valve
Senses modulator pressure to apply a variable force that tends to hold the 2-3 shift valve downshifted.

3-2 Valve
Shuts off modulator pressure from acting on shift valves after direct clutch has been applied. This allows fairly heavy throttle operation in third speed without downshifting. In third speed detent pressure or modulator pressure above 105 psi can be directed to the shift valves to provide downshift forces.

Governor

The vehicle speed signal to the transmission is supplied by the governor which is driven by the output shaft he governor consists of flyweights and a regulator valve. Centrifugal force of the flyweights is imposed on the regulator valve, causing it to regulate a pressure signal that increases with speed.

Governor pressure acts on the modulator valve to cause modulator pressure to decrease as vehicle speed increases. Generates a speed sensitive oil pressure that increases with output shaft or vehicle speed. Governor pressure is used to control the shift points and modulator pressure regulation.