92 Olds 88 speedo problem,fuel consumption high, and speed sensor

I have a 92 Olds 88, 3.8L with about 70,000 mi. The fuel economy used to be great. Now it used about a third of a tank of gas after going only 65 mi. At the same time the speedometer seems to be flakey.

Does the Vehicle speed sensor(VSS) supply the information to the speedometer? I've heard if the VSS sensor goes the fuel economy drops a lot too.

How do you change a VSS? It's behind the right side of the engine but I haven't sent one before.

Thanks for the help.

Sirdeeker