Speedo inaccurate

[Dylan Durr]

My vehicle speed is higher than my speedo. Of course the first thing was to change out the Vehicle Speed Sensor. Same issue. 15-20 mph difference between the truck and speedometer using a gps on my phone.
Pulled out the pinion gear for the speedo at the transmission tail housing (1994 dodge ram 2500 automatic 47Rh)
Looked at the gear. Gear was green, and looked almost new. Reinstalled gear, and made sure to index it correctly. No change in speed.
Checked automatic transmission fluid making sure it was at recommend level.
So where do i go from here? Bad speedo cluster?
OD locks up still at 45mph according to my gps.
And kicks down when i floor it...

 

[fest3er]

Dunno how different '94s are from '98s. What follows may apply partially or not at all to your '94.

On my '98, the rear wheel sensor feeds through the CAB (Controller, Anti-lock Brake) which send a signal to the PCM. So a 'failing' CAB is a possibility.

But then, my '98 is now also registering low miles (10%-20% lower than actual). The speedo may also be low; OD engages at about 40 MPH now, where it used to engage closer to 43. I haven't yet checked with any form of GPS to verify. Thus it could be a failing PCM.

But I still have the ancient TruSpeed connected (from when I ran 19.5 tires); *it* could be causing problems.

The system counts pulses (as each tooth passes the sensor, a pulse is generated). At very low speed, the rear-wheel sensor signal voltage drops below about 300mV (around 3MPH). As vehicle speed increases, the voltage also increases, to a max of around 2.5V-3V.

An oscilloscope might help you trace the problem.

 

[BigPapa]

The ‘94 doesn’t have the sensor in the diff. It has a gear drive in the transmission. Totally different system.

 

[fest3er]

But are the electronics completely different? or reasonably similar?