Thanks for using Just Answer, form what you have described it sounds as though the clutch slave cylinder is either bypassing internally or is defective.
The speedometer is connected to the output shaft of the transmission by means of a flexible shaft (core), and a drive gear located inside the park brake assembly. The core drives the speedometer which registers speed in miles per hour (also in kilometers per hour). It also drives an odometer which records distance traveled in miles and tenths of a mile (also in kilometers and tenths of a kilometer).
All vehicles equipped with gasoline (EFI) engines and/or automatic transmission have a speed sensor attached to the park brake assembly. This is used to sense vehicle speed required for the engine/transmission management system and to regulate the speed control system for vehicles so equipped.
2.Disengage the cable assembly from the park brake assembly and remove from the transmission.
NOTE: Remove the speedometer cable by pulling it out of the speed sensor. Do not attempt to remove the spring retainer clip with the speedometer cable in the sensor. To install the speedometer cable, align the core with the sensor and snap cable assembly into the speed sensor.
3. Disconnect the speedometer cable from the speedometer.
NOTE: The speedometer cable routing should avoid sharp bends; the cable should be straight for approximately 203mm (8 inches) from the speedometer.
You may just have defective speed sensor.
The speed sensor is located on top of the rear differential.
The programmable electric analog speedometer receives its input from the anti-lock brake sensor except on F-Super Duty Motorhome Chassis where it receives its input from the park brake-mounted speed sensor. The liquid crystal display (LCD) odometer is a combination trip odometer and total odometer. The total odometer is normally displayed. To see the trip odometer, press and release the SELECT button on the upper right side of the speedometer. To zero out the trip odometer, press and release the RESET button on the upper left side of the speedometer while the trip odometer is displayed. If the trip odometer is displayed, press and release the SELECT button to return the display to the total odometer value. The speedometer, containing the trip odometer and total mileage odometer, is serviceable only as an instrument cluster (10849) assembly.
Federal law requires that the odometer in any replacement speedometer must register the same mileage as that registered on the removed speedometer/odometer. Service replacement speedometer/odometer and odometer modules with the mileage preset to actual vehicle mileage are available through Ford Electronic Service Centers. In nearly all instances, the mileage continues to accumulate in the odometer memory even if the odometer does not display mileage. This mileage can usually be verified by the electronic service centers. Contact the service center for instructions to receive a replacement speedometer/odometer or odometer module with the mileage preset to actual mileage.
If the actual vehicle mileage cannot be verified, the service center will supply a speedometer/odometer module with the odometer display preset to zero ("0") miles and the service odometer segment ("s") illuminated in the vicinity of the odometer display. In addition, an odometer mileage sticker is supplied with the replacement odometer. This sticker must display the estimated vehicle mileage and is to be affixed to the driver's door.
NOTE: Before ordering an instrument cluster (10849), contact the instrument cluster technical hotline for diagnostic and system troubleshooting assistance. The hotline phone number is XXXXX
The speedometer is also used to provide a speed signal for correct operation of the vehicle's powertrain control module (PCM) (12A650), transmission (7003), and speed control (if so equipped). Because of this, it is VERY IMPORTANT to change the speedometer's internal conversion constant if the size of tires on the vehicle is changed. Changing the conversion constant to the proper value when the tires are changed will also make sure that the speedometer retains the original factory-set accuracy. The conversion constant can only be changed six times once the vehicle leaves the factory. On F-Super Duty Motorhome Chassis the conversion constant must never be changed. If tires or rear axle ratio is changed, the gear in the transmission drive gear must be changed.
Yes if the truck has an older transmission. Prior to 94.
Yes but you will have to change so much, it seems more logical to just repair the problems you are having now.
That may be the whole problem, yes.
Here you go