Which way is actually faster if you utilize the legality of driving fast on the German highways? The A9 has many zones only governed by the advisory speed, but there's so much traffic on it you're lucky to average 100kph. Is the A7 more open? I'm usually no further north than Frankfurt, but I have to say the A14 and A2 were almost as good as how driving in Germany was 25+ years ago.

The A7 has looong stretches with 120 and even 100 km/h speed limits, notably throughout most of Niedersachsen first leaving Hamburg, then between Egesdorf and Schwarmstedt (65kms), between Hildesheim and Melsungen (100 km) through the "Kasseler Berge" (that stretch used to be very fun to drive 30+ years ago with no limit and no traffic!) and between Aalen and Ulm (45 kms).
And traffic is actually worse than on the A9. The A7 is the central north-South route linking Scandinavia and Southern Europe.Tons of trucks, and in the summertime tons of slow-moving vacationer traffic.
Go on a Sunday (no truck traffic) and start very early to make the best of it.

Here's a map listing all the stretches of autobahn with no fixed speed limits. It's pretty accurate IME:
https://www.google.com/maps/d/embed?...9803115698&z=7

Except for the A61 west of Bonn - variable speed limits only, but always set to 130 or 100.

There's always the A7 - A2 - A36 - short bit of B180 - A39 - A71 - A73 - A9 combo to do Hamburg-Munich. Avoids the worst of the truck traffic.

Good call on A9. A7 was moving but quite busy, A2 A14 A9 was an open road as far as modern Germany goes.

I would defintily recommend A7. Usually less speed limits and more lines.