Custer State Park to West Yellowstone

I took that route with my FW about three years ago and would do it again in a heartbeat. The Big Horn Range was beautiful and we saw our first moose of the trip coming down the west side.

X2 on what mabynack said. We have driven over the Bighorn Mountains 3 times, twice on route 14 and once on route 16, all in the eastbound direction. Route 14 is steeper and has more curves, but IMO much more scenic. There is a large pull out on the left going up the east side of 14 that has a great long distance view of the plains to the east. Going down the west side is Shell Falls, worth a stop. We saw Moose both times we drove 14.

If you want to break up the trip, there are a number of Forest Service Campgrounds in the Bighorns. We stayed a night at Tie Flume two years ago and loved it.

16 to moorcroft is fine. at buffalo you can choose 14 or 16..both are scenic.

if you take 14, find a campground near Burgess Junction and spend the night at least. Use the time to drive your truck up to the Medicine Wheel which is west of Burgess Junction several miles. If you like old indian stuff, this is a good one.
If you use 14 to descend toward Cody, stop at the Shell Creek rest stop..nice one.
If you use 14A, be prepared for a VERY steep grade of up to 14%...yes, I said 14%. :) It's a spectacular road, but only for those with good brakes.
And it will be hot in July, so do watch your brakes and transmission.
If you use 16, there are several campgrounds along it, too, with an rv park in Ten Sleep on the west side of town.

Directly west of Custer US16 is an easy drive through the Black Hills, with a narrower section the Jewell Cave - and there was/is some recent construction to improve that.

US16 in eastern WY is as plain as I90. There's more scenery if you detour north to the area around Devils Tower.

Across the Big Horn Mtns all the roads are scenic. Us16, US14, US14A ranks them in order of increasing difficulty.

Then there's another open area till you get to Cody.

Finally a gradual climb to the east entrance to Yellowstone.

For more serious scenery and grades consider the NE entrance, which can be approached from Cody via the Chief Joseph Hwy.

Thanks for the feedback.