The digital readout on my speedometer started fluctuating five mph at steady speed. Irritating, plus I wasn't sure exactly how fast I was going.
I figured the speed sensor pinion gear went bad or was worn. A new one from the dealer was less than thirty bucks, but they could not sell me a new one until I pulled the old one out and counted the teeth on the pinion gear.
Instead of pulling the whole sensor out and having my manual tranny fluid run out onto the garage floor, I just unscrewed this collar where the speedo cable screws into the rear passenger side on the transmission.
Squirted both sides with penetrating oil, screwed back in place, speedometer works perfectly again.