Cluster acting weird

[Skynet]

I have a 1990 S13 hatch, and the gauge cluster is acting very odd. The tachometer doesn't work at all, however the speedometer Sort of does. When the car is completely off, or at Idle, It stays at 10mph. Once you drive the car, and actually get passed 10mph, It does go up to the appropriate speed, But if you decelerate at all the speedometer drops VERY slowly, ie; accelerate to 40mph, decelerate to 25mph, the speedometer reads around 35mph. Any ideas on what the problem could be?
(I searched, Wasn't able to find this problem on the forums)

[Cone Killer]

Sounds like you have two separate problems. One would be just a bad tach, either the gauge or the connection. The other is the speedo sensor or speedo head. My speedo did the same thing, if your odometer is working and accurate, then you probably have a bad speedo head. If the odometer isn't working or isn't accurate, then its the speedo sensor in the transmission is bad. To check your odometer, just drive on the highway, and see if the odometer matches the mile markers.