The tachometer on my boat started going haywire last season. It reads 4500 rpm when it should be around 2500. It stays at around 1500 rpm when l shut the key off. If l turn the key back on it sometimes goes back to 0. Wires are all tight. Could it be a short or should l spend the $70 to $80 for a new tach?

This sounds like it is time to purchase a new tachometer. If your wiring has not changed and all connections are good that would rule that out. Tachs are electronic equipment and on a boat even if your careful they will get wet. Make sure the new one will work with your engine, tachs will have setting for the number of cylinders. Make sure that is correct on the replacement or your readings will not be accurate.