Speedometer problem eratic

1997 escort

Just bought it and installed motor and trans, now I have a speedo that only reads up to 48 mph at times.

I have noticed that in the morning on the way to work the speedo works perfect, 15 mile drive. Then when I go to leave work, as i pull away it climbs at a rapid rate untill it reaches 48 mph, even if im going 10 mph. I stop at a light it very slowly drops to zero. I'm asuming it has something to do with the outside temp for some reason, because it works fine in the morning (it's cooler) then at the end of the day it's acting up (it's warmer) I know it sounds stupid but thats the way it is happening im pulling my hair out trying to figure this out:banghead:. I'm thinking maybe the board on the back of the cluster, i havent pulled it out yet. does that sound reasonable?
oh yeah the odometer is right on to the tenth so i'm assuming its not the sensor.

Any help would be great
Thanks

Welcome to AF!!

I would change the speed sensor first before messing with the cluster. It will be easier and I would bet it will fix the issue.

Update speed sensor didn't fix it:

I although did replace the speedo cluster this did fix it.

I took it apart to change the odometer back to the original miles and found the pins that connect to the board severely corroded, that would be why it only acted upo when it was warm.

Now i know how fast im going

thanks