[shovel]

FWIW, before I modified my 4.3 s10, as long as I had 1/4 tank of e10 I could top off with e85 and stay within the stock ECU's +25 fuel trim bandwidth.

On my 4.7HO jeep, fills-up from empty on E85 keep me inside +20 fuel trim on the stock ECU (trim runs at -1 to +1 when I fill from empty with E10) - but the HO engine has higher flow rate injectors.

It should be noted that the stoichiometry difference between ethanol and petrol has the natural benefit of reducing pumping losses - specifically since the fuel delivers some of the oxygen necessary for combustion, less air needs to be drawn through the engine for a given level of output. So as has been noted elsewhere, between the timing benefits of a higher octane fuel and the reduced pumping losses of a more oxygenated fuel, there have been reports of improved MPG on blends despite the lower BTU count per unit of volume.