Alan Taylor has collected the longest drives that Google Maps will give driving directions for.
It turns out there are multiple "longest drives", because the Google Maps World is partitioned (many countries don't support driving directions), and sometimes ferries are included, and sometimes they are not.
The longest he's found so far is from the Aleutian Islands to the tip of Newfoundland, a distance of over 7,200 miles. You can drag the path around to make it a lot longer (more than 11,000 miles) but that's cheating.