For your speedometer, it could simply be the cable is broken. As for the tachometer, I would check the black cap unser the hood. Looking at the engine, over on the right hand side on the wheel well, you will see a round black plastic cap. If the cap is nice and tight, then the problem may be inside. There is a little trick you can do using a cigarette butt, or something similar. With the cap unscrewed, you can gently take the cap apart and insert a cigarette butt inside and put the cap back together. What happens to these is the connections get weak over the years and it's not staying seated properly. So by placing a butt or something similar behind it, applies pressure to keep in seated.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section