Do you have a check engine light? If so, get it scanned by a repair facility to make sure you don't do any unnecessary repairs. It could be many things, such as an o2 sensor or throttle position sensor just to name a couple. If you don't have. Check engine light, sounds like a faulty engine control unit. Again, have someone scan it with a computer. It will save you money in the long run.
replace the ignition coil. There is a resistor inside the coil that fails over time and causes 2 problems. 1 the transmission may fail to properly lock the torque converter and 2 will cause the tachometer needle to fluctuate wildly, esp when it is hot outside.
The tachometer cable may be faulty. The tachometer sensor, on the side of the engine might not be functioning properly.
due to contraction of heart
Cruise will NOT work if the speedometer has a problem
A bad/dirty vehicle speed sensor (vss) or bad wiring will cause this. the sensor is usually a magnetic pickup and any metallic dust stuck to it will affect the signal clean the sensor or get a new one
Temperature.
I need a fuse diagram, our speedometer does not work, neither does our seat belts work automatically. The drivers side seat belt is locked in the off position. We felt it may need a new fuse. We have changed the instrument cluster on the odometer.
Broken speedometer cable.
Could be low on coolant or a sticky thermostat.
Is the TCM working normally and throwing no codes?
When you turn a corner the guage fluctuates.
If the speedometer and odometer are both not working, I would blame the speed sensor. If only the speedometer does not work and the odometer does work, the cluster is bad.