I am seeing a consistently small increase in fuel mileage from burning up old gas. The background is a friend dropped off a Bronco that was sitting in covered storage since the year 2000. It had gas in it, but it stinks and looks quite brown like tea. I set aside 5 gallons and have been slowly pouring a quart or two every time I do a long trip. My car is ford fiesta with fuel injection and a digital fuel mileage readout that averages fuel flow versus mileage driven since its been reset. It doesnt have a turbo so probably no knock sensor. The really odd thing is I see a consistent increase in mileage by a couple or tenths to a half a mile per gallon. I usually do very long business day trips of 400 to 450 miles. I add the old fuel before the start of the trip and then refill the tank before heading home. its pretty consistent that the mileage will drop back a couple of tenths when I drive home after diluting the original tank and if I do not dose it as I am in rush and do another trip without adding the old gas, the mileage drops back a few more tenths and it stabilizes. This is definitely not a scientific study but is pretty consistent. I use cruse control pretty consistently and usually set it pretty consistently. My speculation. Year 2000 gas is probably non ethanol and may have some additives like MBTE that are now banned. Any volatiles had long since gone out the tank vents so what is in the tank is the lower fractions of what once were gas. In theory this has a high btu content along the lack of ethanol. I am adding a quart or two to a roughly 10 gallon tank so I could just be raising overall btu content. I thought the old gas may be acting like octane booster but since there is most likely no knock sensor this discounts the octane theory. Once I get rid of this old fuel, I may try to blend in small amounts of non ethanol gas to see if I get the same effect. Note I only do this on long trips to minimize potential injector clogging. So anyone care to speculate?