Speedometer trouble

So last summer my younger brother bought his first car, a 1991 Camaro RS. It had some problems, but the price was definately right and he seemed interested in taking the time to learn about the car and fix it up. Just about all of the problems have been worked out and the car seems to be running great except for one small problem....the speedometer doesn't always work. I've never had speedo problems on my Camaro, so I couldn't really offer him any advice.

Here's what I know:
The speedometer generally doesn't work and the odometer doesn't gain mileage.
While attempting to diagnose the problem my brother removed a "small red thing with a gear" that was attached to the transmission. He spun said red thing, (the VSS perhaps?), and apparently was able to measure a voltage across it.
Lastly, once the car has warmed up the speedometer will start working, but only while driving downhill.

Any advice is appreciated!

He should check and make shure the speedo cable is plugged into the spedometer correctly.

I finally got home from college so I should be able to offer my brother some more help. We havent checked the speedometer cable yet, but I'll get on that. Another noteworthy event, the tach is definately wrong. The car generally drives around 3000RPM, (even though my ear guesses its closer to 1500RPM), and will rev as high as 6000RPM when accelerating, (really I dont think it has gone past 3000 though). What would cause the tach to read out a blatently wrong value?

I don't think your 91 Camaro has a speedometer cable, by that time they had gone to electronic speedometers like the Firebirds had starting in like 87. This is driven from a signal from the VSS located in the transmission where the speedo cable would normally hook up.

As far as the tach goes, all 3rd gen tachs are junk, they start to go out at about 10 years old and when they do they always read way too high. You could get it repaired (google it) or try to find a functioning tach from a junkyard and use that.

My camaro is an 89 and it has a speedo cable...
Its true that the tachs can have problems...mine never has...but the genneral consensus seems to be that they develop issues.
I would start by pulling the cluster and checking the electrical connections it plugs into and also check them for any obvious dammage...
I would suspect that there is an electrical issue somewhere...probably a dirty or corroded connection somewhere that is causing the voltage signal(the amount of electrical current going to the gages) to go out of speck. If this is the case, then there is nothing wrong with the gages, they're just displaying the info given to them and that info is wrong.

Go around and clean any and all electrical connections you can find(under hood and dash) and check wiring for damage. Remove the relays in the engine compartment and look at their underside for heat damage, if there is any, then replace it. Also clean the electrical grounds on the engine.

Let us know if this helped.