NotAMechanic

2005 Tahoe
I disconnected the battery to remove the windshield washer reservoir and after finally getting it all put back together, the vehicle started and ran fine. At least until I drove it. After a short time going down the road the speedometer began to show about 20 mph (?) over what I was doing. After a couple of stops in town and back on the hiway, it began to show WAY over what I was doing. Staying with the flow of traffic which was about 60, I was showing 110 on the speedometer and about 1700 on the tach. At a stop it still showed +/- 55mph speedo & +/-500 on the tach.
The question is do I need to have this vehicle reprogrammed? Or is there something I can do at home to fix this? Everyone that I have mentioned this to says that they have never heard of this. The only oddity that I can think of is the wires for the rear washer pump pulled out of the socket, but I got them put back and the pump works fine when I push the button for the rear washer/wiper control.
Thanks in advance for any help and ideas anyone may have.