Old-style air core gauges, or the newer stepper-motor type?
The stepper-motor guages are NOTORIOUS for failure. Fortunately, a simple and inexpensive "fix"--replace the stepper motors by soldering new ones onto the instrument cluster.
My '88 has the old-style pre-stepper-motor gauges. When the speedo quit, the odometer also quit, as did the cruise control, ABS, and the torque converter clutch.
When the speedo came back to life, so did all the other accessories. The speedo and accessories were intermittent for about two years--they'd work sometimes, they'd quit working for a mile or fifty. It was totally random and entirely un-predictable.
I replaced the speedo, with no improvement.
Eventually, I put the original speedometer into a replacement (used) instrument cluster--bingo--works perfectly ever since. The instrument cluster has ICs that take the signal from the vehicle speed sensor, modulates that signal and sends it to the cruise control, speedometer, ECM, etc. With no signal from the VSS, the speedo, ECM, ABS, and cruise control are helpless.
The ECM would control the trans shifting on your vehicle, too. Connect a scan tool, drive the vehicle, and see what the ECM is getting for VSS information.