Speedometer going crazy not reading correctly

Sounds like the speedometer is faulty. If the speed sensor were the problem, the odometer would not work correctly, the transmission would not shift correctly, and the check engine light would be on.

It's probably your speed sensors or your whole speedometer cluster

could be the vehicle speed sensor, it sends signal to speedometer also tells the computer how fast the vehicle is going to properly select the correct gear for the speed.
bring it back to the same mechanic his work should be warrantied

i have a 2005 monte carlo 3.4 v6 auto with overdrive. i recently replaced the whole cluster because the tach and speedometer was reading crazy.i bought one at a salvage yard installed it and have had no problems for several months.now unfortunatly i am ee=xperiencing the speedometer dropping speed all of a sudden and then it will go to zero and not change at all.the only thing i can think of is the fact that i installed a set of 18 inch wheels and the same day my speedometer started the crazy stuff.so if you have the wrong size tires or wheels i assume that this is the problem.the computer is reading something wrong and the speedometer is responding to this.i hope i have helped you.

please keep me posted as well, ALL my gauges are doing the same. First the gas, filled it one day and it fell all the way and light turned on, then my themostat, freaked me out, thought i was overheating,...NO just the guage, NOW my spedometer is stuck on 120, and my RPM's are acting crazy. i have read tons with this same problem, WHY WONT the manufacturer DO SOMTHING??

First I would go back and check out the turn signal switch to make sure you install that correctly. Look for shorts or wires that could be grounding out. Go to the library and getting a wiring diagram and find out everything that goes to that fuse. Also if the truck is rusty check all the body grounds. Good luck.

Have your altenator checked. If your battery isn't being charged up enough from a faulty altenator the low voltage will make your car go CRAZY!
Best thing to do is buy a Haynes or Clinton's repair shop manual at your local AP store for your particular vehicle. Then read up on it in the trouble shooting section or the step by step RR section. You can purchase these at your local Auto Parts store or buy them on line at ebay burdfrenzy

open curcuit in crank sensor replace it.

the speed sensor in the trans could be the culprit I would have the wiring inspected first though

My guess is that the speed sensor has gone out or the wires that go to it have been knocked off somehow.