Cruise disengages and speedometer flutters

I have a 2002 Kia Optima, 6 cylinder, with 157K miles. The cruise disengages and when that happens, the speedometer flutters back and forth from 0 to 80. The engine speed doesn’t waver, it just slows because my foot isn’t on the gas. It doesn’t disengage every time I use it, but when it does occur, it will happen several times in a row. I have replaced the cruise control unit, but that didn’t solve the problem. I have also noticed that it may be happening at the point of shifting up to the next gear, but I’m not sure that the two events are connected. I’d appreciate any insight you can give me. Thank you.

It is unfortunate that you already replaced the cruse control unit, which probably was working perfectly.

A car will automatically drop out of cruise control if the computer thinks its speed has dropped too far. Since both the computer and speedometer can only judge speed from the input of the VSS (vehicle speed sensor), my guess is that you have a flaky VSS.

I agree. The vehicle speed sensor is likely to to be the problem. Too bad you already spent the money for a new cruise control unit. Was that your decision, or a mechanic’s?

Does this car have an automatic transmission?

Check to see if this car has a flex drive cable from the transmission to the speedometer. If it does a worn cable can cause the speedometer to jump around. Expecially if the cable is dry. May just need to lube it or replace it…

The mechanic’s.

Yes. It can also be shifted manually.

the question was does this car have an automatic transmission. it has either a manual (stick shift) or an automatic.

I don’t think the type of transmission is relevant. Manual, automatic, or even CVT, the symptoms all point to a malfunctioning VSS.