1st, clean and set your ignition points.

I suspect the ignition distributor. It has a timing retard mechanism, I believe in the '72, if injected. Sounds like maybe this isn't working. I doubt, without serious other symptoms, that your engine is actually running slower at highway speeds. Since the tachometer is signalled by the ignition, usually through a distributor connection, I again would suspect connections at the distributor.

Basic things to do:
Remove distributor cap, inspect for dirt, corrosion, clean. Clean points. Cceck and clean all electric and vacuum connections. COuld be the vacuum retard mechanism.

Less likely, although possible, if D-jet injected, are the lower set of F.I. trigger points in the bottom of the distributor, although these usually work or it only runs on two cylinders.