You will be hard pressed to find a deserted Autobahn around Frankfurt. The drive to Nurnberg via Wuerzburg is OK, or taking the AA3 in the other direction heading up to Cologne as great, too. Particularly up there the Autobahn is like a 6 lane rollercoaster.
For decent rental cars I highly recommend Sixt (
www.sixt.de)
Drive Carefully!
Bochaco