Map of US highway 91 and family, and US 163
US 91 is represented by the purple line from Idaho Falls to Brigham City UT. The red lines extending north and south of those points are the historic extents of US 91.
US 191 is the blue line from Loring MT to Douglas AZ (including an implied route through Yellowstone National Park). The original route of US 191 from West Yellowstone to Idaho Falls is shown in red, as is its historic routing north of Brigham City and west of US 91.
US 491 is the blue line from Monticello UT to Gallup NM.
US 163 is also shown on this map (the green line from Bluff UT to Kayenta AZ). Because its number is a major violation of US route numbering conventions, I included it on this map to demonstrate how the highway would fit into the US 91 family, if it were to be assigned a number US x91. The red dashed line north from Bluff to Crescent Jct. is the historic extent of US 163.