I have a 2000 Mazda B3000 cabplus pickup and recently I noticed when I am on the highway and I am accelerating around 60 and over the speedometer jumps all over the place (sometimes 50, sometimes 0, sometimes 60) it never reaches the proper speed that I am going. Once I take my foot off the gas the needle then rises to the proper speed limit (say 70/75) and slowly decreases normally until I put my foot on the gas and accelerate at a higher speed. It will then jump around all over the place again. What can be causing this issue?

Thanks for your help in advance

 

It may be the VSS (Vehicle Speed Sensor) according to info I've found for a friends f150. I've got a 93 B2600 5spd that is starting to have the speedo bounce a little while at speed when normally it is steady. The dealer is saying mine might be called a revolution sensor but I don't know for sure.