78, let me know if you need any details. Being that I'm a South Dakotan and been there many times. Just give me a call, send me an email, or post a question here.
Suggestions:
Take the Spearfish canyon loop from Spearfish to Savoy (look at the Roughlock Falls at Savoy), Then on to Cheyenne Crossing, Lead, then Deadwood. Once done in Deadwood, head south on 383 or back track on 85 to 17 South and head to Rochford (nice drive, but hard surface only one way in). 385 South to Hill City (stop by the Black Hills Museum of Natural History and check out the dinosoars and stuff) Yes, South Dakota has had more T-Rex fossils then the rest of the world! Then Many ways south to Keystone to see Mt Rushmore. (ALT16 is the best way to see Mt Rushmore, but take it from the South heading North... And Hot Springs is a lower elevation town with Wind Cave National Park right North of it.
Enjoy!
Scott