Heck all the places you mentioned are actually not far enuff out of your way to worry about the extra fuel. Hit Rapid City and down to Mt Rushmore and then Custer,Newcastle and then 16 back up to Devils tower and then back to 90 and over to Yellowstone. You're going to probably do the Badlands-Wall loop on the way.Stay in the park if you can get res at Fishing Bridge and you won't have the drive in from West Yellowstone every day. Man, I saved a LOT of time and gas by staying there and doing my day trips. If you have a tent or p/u or hybrid,you can't stay there tho.
