The first time it happened was about two months ago...but it fixed itself, so I ignored it. But it happened again today so I figured I'd ask.
I'd start driving...and all my gauges work, excpet the speedo.
The first time it happened, I tried smacking the top of the dash..nothing. So I pulled over, crawled under the truck to make sure the wiring was still connected, and it was. Started back up and driving, still nothing. So I decided to ignore it and use the tach for my speed. About 4 miles later it started working, while I was driving.
Exact same thing happened today except I didnt bother stopping. Speedo kicked back in about 4 miles later.
My guess....bad tooth on the speedo gear?