The speedometer quit working and it's messing with the transmission.

After a long haul pulling a car and trailer my speedometer quit working. It seems to be messing up the transmission on highway speeds telling it to shift gears when it shouldn't be. My job relies on this vehicle and would like to attempt to fix it myself if you have any advice. I can't get in to see a mechanic right away and I'm scared I'm going to either break down or get pulled over because I don't know how how fast I'm going!