If you were traveling by vanagon, and did not care for superhighways, steep passes and headwinds, which summer route would you take from Eugene, OR to Yellowstone?
I don't know about passes and headwinds, but avoiding super highways (by which I assume you mean interstates), you can go from Eugene to Yellowstone National Park on US 20 to Burns then Oregon 78 to US 95 where you cross the border into Idaho. Then Idaho 78 to Walters Ferry (Idaho) at Grand View Idaho switch to Idaho 167. In Mountain Home (Idaho) take US 20 and US 26 to Idaho Falls and then up to West Yellowstone
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