autometer speedo "walking"!....

I got my florida 5.0 cluster in a few days ago and each time I start the car, the speedo creeps up to 20mph with the car sitting on 4 jackstands! Any ideas as to why? The cable is already hooked to the speedo gear and the driveshaft is NOT hooked to the tranny yet. Honestly, each time I've used an aftermarket speedo in any project I've done, they've never been accurate. The one in my old 302 swap efi 86 F150 would walk a little and was off by 8 mph at cruise speeds.
It annoys me that this happens but a speedo is needed to have SOME sort of idea....

NikwoaC said:

It is possible that the output shaft of the trans is spinning with it in neutral. With the engine running, foot off the clutch, trans in neutral, the input shaft is spinning. With nothing to stop it from turning, it makes sense that the slightest bit of friction in the bearings between the input shaft and output shaft, the output shaft could start turning with the input.

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wow, never knew that could happen. Had this car since it came off the truck and still learning stuff! The clutch cable isn't in either. I guess once i get the driveshaft in and the clutch cable all buttoned up, it'll cease. Thanks for the info!