The gas and temp gauges are both powered by the panel voltage regulator,
so if it croaks, they would both go off. The one common thing for the
tach might be the +12V supplied by the green wires that power the tach
and the panel voltage regulator. If there is no power in this, then all
three instruments don't work. If it was the fuse, then some other
stuff, wipers and brake lights for instance, also would not work.
Or you could have two or more problems that just happened to occur
together, but this is less likely than one problem that causes it all.
Stu
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