If you are correct about the tach suddenly dropping, it isn't the main
relay. If the fuel pressure dies the engine will run down over a period of
half a second or so while cruising down the road or over a couple seconds
while idling or coasting (the voice of experience!) In fact, just the time
it takes for the engine to stop spinning is a lot longer than it takes for
the tach to just drop, so when you see the tach drop like a stone you can be
certain the ignition is dying.

You don't mention whether it is an auto tranny or manual gearbox, so I'm
assuming it is auto. The difference is even more pronounced with a manual,
since the engine keeps turning until you depress the clutch. In that case,
the tach just follows the engine speed down if the ignition is working and
the fuel quits.

Mike