Speedometer inop/fluctuates

My 13 speed will show 30-35 miles per hour on the 'splits' or low side. I was just using my GPS mileage for the IFTA reports and let the truck track fewer miles than reality- better for resale value later.... lol But I heard there is a speedo/odo wire coming from the transmission that might need replacing.

 

The speed sensor itself could be your issue. It's the quickest, easiest fix, and your symptoms point that way. On that old of truck, a tailshaft bearing getting loose could also cause this by allowing the tone ring to move away from the sensor.

 

Look for evidence of rust around the trans output yoke nut. If the yoke is loose the tone ring for the sensor can spin on the output shaft and cause the speedo to drop out or read lower than normal. A crude method of checking the wiring is to get a new sensor and plug in to the existing wiring connection at the VSS in the tail-housing. These are cheap like 40 bucks. Lightly and rapidly tap the sensor on any metal object under the truck. Have a partner watch the speedometer with the ignition on the speedo should bounce while tapping the sensor. It will only hit maybe 5-10mph. If no bounce, you likely have a wiring issue somewhere. If you have good signal you indeed likely have a bad Vehicle Speed Sensor. Be prepared to replace the trans output seal when changing the sensor. They get rust jacked inside and the housing will need to be removed to grind out the old sensor and the rust buildup.