I'm with Nigel on the autobahn. I find driving it very stressful because you have to be actively engaged and I hate hate hate the reckless behavior of the truck drivers.
Regardless though, a road trip from Munich to Berlin with only 8 days doesn't make much sense to me at all. The distance between the cities is great enough that a you'd be shortchanging both cities to have the road trip be worth it. I mean, 2 full days in Munich and 3 full days in Berlin are the absolute minimum you'll need to hit the major sights.
Day 1: Really a half day depending on time of arrival, jet lag, etc. Berlin.
Day 2: First full day in Berlin.
Day 3: Second full day in Berlin.
Day 4: Third full day in Berlin.
Day 5: Road trip
Day 6: Road trip finishing in Munich
Day 7: First full day in Munich
Day 8: Second full day in Munich
See what I mean? You could MAYBE shave a full day of Berlin but it would be a shame. So you'll end up with just 1-2 nights road tripping across what's a 6 hour drive if you don't encounter significant traffic. I just don't see the point.
Honestly if you want to road trip in Germany I'd pick a more scenic, smaller route so you could take the B roads and meander. Plenty of places in Bavaria lend themselves well to scenic road tripping. I'd spend a few days in Munich then do that instead.