What could be doing this?

I got my clutch plates replaced on my silverado 4x4. Picked it up yesterday not far from the house. Today I drove it more and it seemed to not go into the highest gear and when it was saying 45 it seemed faster than that. I cut on the cruze control at 40 and it was giving gas then slowing down and kept on doing that. I am going to take it back to the shop to have it fixed but I would like to know what it might be before I do. Anyone with suggestions? Thanks

I’m assuming your talking about having an automatic transmission rebuilt, it will not go into over-drive, and the speedo is off. Yes, you need to go back to the shop. There is a problem, and it may be as easy as a bad connection for electronic shift transmissions, or a valve body problem in a mechanical shift tranny.

Yes its a automatic. Thanks.