Interesting, yes. My bets would be spread like this:

1) Onkel Udo II and Jorrell are correct about a ground or shared ground. (At this point you may just agree this is academically interesting only)

2) The decoupling cap in the tach dried out 10 years ago. (You might replace a 47 microfarad cap in a spare tach and swap it in...)

3) The tach wire parallels the horn wire between cluster and coil area, and the horn is electrically very noisy. (You can smile at the Volvo engineers and check to see your horn is a real Volvo horn and not one of those made-in-China, wound with 10-gauge imitations.)

4) The tach is sensitive and the sudden use of the horn startled it. (Time to give your 240 a name.)


--
Art Benstein near Baltimore

It's always darkest before dawn. So if you're going to steal your neighbor's newspaper, that's the time to do it.