So, we've been wondering...   How exactly did monroy get his tanks approved?   91.205 says you have to have a fuel gauge. The old wives tale is that they have to be accurate when empty and full, which the monroy tanks aren't.   However: how do they comply with 23.1337 exactly? http://www.aviationlawmonitor.com/2011/06/articles/general-aviation/aircraft-fuel-gauges-are-supposed-to-actually-work/   § 23.1337 Powerplant instruments installation.