I took 421 from the Fayetteville area to Tennessee. I read that it was mostly 4 lane and it was - UNTIL I reached the NW corner of North Carolina.
Then at Boone it started over the Appalacians Oh Oh!
It was 15 to 35 mph with the longest straight away of maybe 100-150 feet! Constant S curves and hairpin curves. Finally made it thru - or so I thought. Got to the valley and a town called Shady Valley. After about 1 more mile I see a sign that says 'Curves and switchbacks next 10 miles'!!! So I start again and notice I have 1/8 tank of fuel - oops. At the top, the Low Fuel light comes on. I get about 11 mpg on the level, not sure about the up and down in the mountains but certainly less than 11. And I have another 5-6 miles to go. Well I made it - at the bottom within a mile apart were 2 gas stations!
So the moral of this story is don't take 421 into Tennessee!!