The speedometer needle broke on my 81 XJ 650 due to age and exposure to the sun. The Yamaha dealers said new speedometers are not available and the salvage speedos i looked at were no better than the one on my bike. I carefully disassembled the speedometer. I found a spray can with a red plastic top and cut a needle from the side of the top (so that the needle would have a little roundness for strength). I used some automotive RTV silicone to attache over the top of what remained of the factory needle and let it set for 24 hours. I have put 2,000 miles on it since the fix and it is holding up fine. I recently had my forks rebuilt and the shop owner wanted to know how I fixed it because he has requests for replcaing broken needles. I was not careful enough and have it positioned so the speedo reads 5 mph faster than the bike is going.
This fix would work for a tachometer as well. 
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