Most (&gt;95% for sure) small planes use leaded fuels. Many (probably three-quarters of the fleet, but probably less than a quarter of the gallons of avgas burned) of those planes could readily use ethanol-free unleaded avgas without significant modification.<p>The high powered planes are the ones that also fly the most hours and burn the most per hour, which is why the bias is so different by gallons rather than fleet size. Many of those perform critical functions (mail&#x2F;parcel delivery, food delivery, remote people transport, pipeline patrol, traffic reporting, pilot training) and there is no certified replacement fuel available. The logistics are such that if an airport business has to carry leaded fuel for 75% of your avgas sales and the other 25% can also burn 100LL, there&#x27;s little incentive to install duplicate tanks, fuel trucks, etc for zero additional sales of fuel.<p>There are promising unleaded fuels in testing (not derived from unleaded autogas) that the high-compression and turbo-charged aircraft can run without modification. The approval process via the FAA is very slow-moving, expensive, and risk-averse. (in many cases for very good reason, but it&#x27;s still a significant barrier in that I cannot legally burn the prototype fuels, nor any unleaded fuels, in my aircraft).<p>I would switch to the GAMI fuel tomorrow if it was available for sale. For the small amount I burn per year (estimated 1500-2000 gallons), even an increase of $2&#x2F;gallon would be no barrier for my adoption and the likely increase vs today is lower than that. There are other fuels also under consideration or certification testing, some of which require airframe or engine modifications, others of which are also &quot;drop-in&quot; replacements.<p>TEL really is fairly magical in terms of improving knock and detonation margins in highly stressed engines, so it&#x27;s no surprise that its adoption became so widespread after World War II. The transition away from TEL is not a trivial exercise, which is why it&#x27;s &quot;taking so long&quot; to make progress on this. The world needs a physically and chemically workable fuel, but also needs an economically workable fuel.