91-2 7's use Yazaki speedo's, and those years are infamous for failure. Some have been lucky with a relatively simple cap change, but others have had more serious issues. Almost all are in the head not the sensor btw, but it's a good idea to prove it/test it. Mucho info in the FAQ.
Some have made entirely new instrument panels, some have adapted panels out of later cars. That last is also in the FAQ with new info I believe, about how to get the fuel gauge to work. Junkyard panels may also be bad, one tip is to check if the trip odo has been reset to 0. If so, it's taken as the first sign that the unit has failed and the PO tried the only thing available, which was the reset button. Mileage on the trip odo may thus be a good sign.
But they are troublesome units.