Speedometer Troubles

I have an '84 W900 Kenworth Tractor. The speedometer is currently running 10 to 15 mph low. It has been acting up occasionally for awhile but usually goes back to the correct speed eventually. I have tried pulling the sensor at the transmission and checking the gear on the transmission and it seems to be tight. The wires at the connector were a little weak so I replaced the connectors at both ends. The speedometer head was replaced with a used one several years ago but it worked fine until just recently. It never stops working completely, it is just about 20% off but it is not always off. When the speedometer is off, the odometer is also off. If you compare mile marker signs to odometer readings when the speedometer is acting up, the odometer reads about 8 tenths when it should show 1 mile. Any suggestions would be greatly appreciated. Thank you!

 

Thank You! I have been using an app on my Android cell phone called GPS Essentials to do that. It is very accurate but it does mean that I need to pull out the phone which is not always the best thing to do no the highway. Thank you! Steve

 

My suspicions were going toward the sensor but I wasn't sure if it would go intermittent like that or if they fail completely without warning. Thank you, Steve