Speedometer jumps a bit during engine warmup

[natjwest]

I have a 2000 Chevy K2500 4x4 6.5 TD with 197k miles and when I start it up after it sits for a day or more, the speedometer jumps up to about 20 mph, up and down for a few seconds. Then it behaves normally. Is this something to be concerned about? It works well (GPS verified) when driving. This problem appeared a few weeks ago.

 

[WarWagon]

Keep in mind how the system works to determine if it is "normal" or not.

The speedometer works by sensing teeth of a shaft rotating by a magnet and the pickup coil. It is possible to vibrate a tooth near the sensor, but, not rotate the shaft more than a 1/16" of an inch and get a speed reading. The park pawl may allow the shaft to rotate some and then drop into park. This can happen on a flat driveway. Engine start vibration will usually drop the pawl in with a slight shaft rotation by the sensor.

Excess vibration and flexing of the engine/trans mounts can allow the shaft to rotate back and forth or the engine/transmission bounce from side to side while the shaft stays stationary.

Wires shorting can also do this. Look at the speed/RPM sensors on the transmission and check for insulation falling off.

Check grounds and battery state of charge before starting. Low or bad battery's cause weirdness as voltages drop and surge when the starter kicks in and out.

 

[THEFERMANATOR]

I wouldn't worry about it myself, it seems to be an issue with feedback from the tach as mine wil ldo it as well. Watch your speedometer and rev the engine up to about 2500 and I bet the speedo will go up to about 10 MPH as well. I know my bURB does this, and my 01 donor truck did as well so it isn't just the older trucks.

 

[duramaxdude]

My 6.5 does that almost every morning dont even worry about it. It looks bad but i got used to mine doing that it doesnt even bug me anymore