Troubleshooting speedometer that is not working

Thanks for the effort and time to post to my speedometer problem. I had thought about the speed sensor but it didn't make any sense because the speedometer was registering speed at idle as well as the economy read-out was registering also. Turns out that the alternator had a bad diode which in turn caused a huge ripple in the alternator's function, that caused the problem with the speedometer. Replaced the alternator and all is well. W. R.