I’m experimenting with pure (100%) gasoline. My last 2 fillups are with 87 octane regular 100% gasoline, and I’m tracking my mileage after each fillup. I calculate it manually (miles driven / gallons) rather than rely on the dashboard gauge which is not quite as accurate.
I’m going to watch it over a few fillups, then compare to my mileage that I’ve been getting using 91 octane “normal” gas (up to 10% alcohol). I suspect that the higher BTU rate of the pure gasoline will at least equal the mileage enhancement that comes with my engine’s adjustment to the higher octane of the 91 I’ve been using (my engine computer does take advantage of higher octane by adjusting the mixture/timing for the slightly slower burn speed of higher octane fuel).
BTW, I always treat my lawn mower gas with Sta-Bil to help keep it fresh and to help deal with some of the negative effects that alcohol can have on the fuel system of some small engines. Haven’t had any problems with my mower, tractor, or weed-eater.
I’ve been down that road with a Toyota Corolla. My experimentation occurred during the first big runup in gas prices, in 2008. Gasoline containing no ethanol does provide better gas milage. At the time it was somewhat difficult to locate without paying a premium for the “white gas” typically found at marinas, but there are stations advertising ethanol-free now, still with a price premium but not nearly so much. I took it a step further and experimented with acetone (the nearly pure kind not paint remover or nail cleaner, those would varnish the cylinders and ruin the engine). After a little trial and error, building up and then backing off, I settled on 1oz. per ten gallons at fill up. That in my experience worked, too, with an overall increase in the 15% range that persisted. Doesn’t work with ethanol in the gas, though. No benefit. Different engines behave differently, so there’s no hard and fast amount as far as using it as a fuel additive.