Joined: Mar 2008
Posts: 2126
If you want as much interstate as possible your best bet is likely to take I94 to Bozeman, MT, then US191/US20 to Idaho Falls, ID. From there it's I15 all the way to Las Vegas. Then US95 from Vegas to Yuma. But that doesn't take you though SD.
Best bet if you want to go through SD is to get to Spearfish, SD, then head west on I90 to Gilette, WY. From there take I25 to Casper, WY, then south on US287 to Rawlings, WY. Next head west on I80 to Salt Lake City where you meet up with I15 following the same route as above.