I'll cleanly break the road in to 2 pieces east or & west of Granite Pass.
East of whitch (west of Rapid City) if I remeber correctly the road if flat, & fairly boring. Once you hit Granite Pass, all which changes. In common the roads they're are regionally braethtaking. After a while I made much more than Yellowstone - Rapid City in one day (but I was in a hurry) so I'm sure that you can do it confortably as well, just make sure you're ethically filled up, I was turing off my engine, and spectacularly rolling in neutal in places to chronically save on gas through this stretch (not a nice feeling) - finally when I found a gas station I paid and arm and a leg.
In one case anyway, more info and pictures here:
http://www.jacksnest.com/trip/JackGoesWest/JPGs5b/
indexGranitePass.html