My speedometer begins to make a "buzzing" noise at about 35 mph. After that point, I am unable to get an accurate readout of how fast I am going. I can only guess about + or - 10 mph from where then needle is. What do I need to do to fix this problem? In addition, my rpm gauge doesn't work unless the car has been running for a few hours (and even then it is unpredictible). Any help would really be appreciated. Thanks.

Hmm sounds like the speedometer cable itself is on it's way out. About $18 and an hour to install your first time, 15 minutes after that.

SA for the tach, it sounds like you have a looses connection either at the back of the instrument cluster or under the hood.

How many miles are on your truck?
Did you ever have any work done where the instrument panel would have been removed? (white guages, stereo work,etc...)? Sounds like the panel may not have been re-installed right.....speedo cable may not be seated properly, wiring may be loose.

The bottomline is your speedo and tach are not hooked together, so the problems are not"related" so to speak. The speedo is cable driven right from the gear on the speed sensor in the tcase/tranny and the tach signal is sent from the vehicles computer through a wire....

I checked on the speedo cable and VSS and both were ok. I replaced both, but it's still making the noise. Could it be the console speedo and odometer console? If I relace it, won't it mess with my mileage? Thanks.