Rep Power: 195

I have a 94 DX. I picked up a gauge cluster with a tach off of ebay from a 93-95 Civic. Didn't specify the exact year. Install was fine. But the speedometer is progressively off. When the speedo reads 35 I am actually doing 31. When the speedo reads 80, I am actually doing 71. I know it is reading miles acumulated faster than I am actually traveling. Is the speed calculated in the actual Speedo guage, or in the cluster housing, the ecm, ???
Thoughts on why it is so far off.

 
Rep Power: 235

I think it's usually the speedo itself that is bad. Maybe, if it was working right, try putting your old speedo in the new cluster. If I'm wrong someone will certainly tell me. lol.