I ran into a German tourist on a rental motorcycle in Hanksville UT. He was having problems with the bike and asked me where the closest repair shop was. I told him it was a tossup between Salt Lake and Flagstaff, each about 230 miles. Sorry, Gunther, this ain't Germany.
afaik the longest distance between gas stations on an interstate is from Salina UT to Green River UT on I70, 107 miles. It's very scenic. There are a couple of rest areas with pit toilets and no water.