Have you checked the fuse? Looks like fuse #7 (counting from L-R) controls the speedometer, not sure about the tachometer.
Whenever there is an electrical item that is acting up (or not working...), it's a good thing to check all ground connections.
Since you tell us the odometer isn't working I'd suggest checking for a loose/broken cable. I believe it attaches to the left rear of the transmission then goes to an sending unit to get converted into electrical signals (for the speedo). The tach gets its input signal from the coil.
Check these threads (the search function found them for me):
Speedo fixed
S4 speedo inop
Elec speedo help
91 speedo not working
how to fix your slow reading 91-94 speedo