That is traditional wisdom, however, there's probably more to it. If your car is tuned for E10 then it would get better mileage from it. Do cars learn fuel the way they learn driver style?

Yes, they do 'learn' the fuel. The timing maps and fuel trims will adjust but that usually only takes the first couple of gallons or at most a whole single tank to make changes if the fuel changes. With E10 the computer learns to richen the mixture out ever so slightly and gives a little extra timing.

Some cars get worse mileage and run like crap because even though the octane is the same the amount of fuel required is a little higher and some fuel systems don't compensate for that either because they have bad O2 sensors or they're carbbed. It isn't THAT much of a change but if the engine is on the edge of running lean as is then you put E10 and no corrections are made then it's overly lean.

I thought that when they add ethanol to gasoline, they do it so that the end result is at the same octane rating as they're selling it -- so they replace other octane enhancers with ethanol. In short, I thought E10 87 octane is still no higher than 87.

Pump octane is a bit arbitrary, some mixtures might burn faster or slower than others, despite having the same "knock" value.

Remember that the octane rating at the pump isn't a maximum number, it's a minimum.

Did a little more reading and they do in fact keep the same octane level. And because of the Ethanol it's possible for the fuel to fall below the rated octane on the pump by as much as 3 points as the fuel ages and moisture gets introduced. We run 86 octane up here but I'd love to run some 84! If it ain't pinging it's not low enough!

I would jack the car up and check front and rear brakes for dragging. He may have gotten the e/brake a little too tight or it is possible you have a caliper sticking. I had my mileage drop like that one time it ended up I had a wheel bearing that was bad. Couldn't tell it in driving and it wasn't grinding, but when I jacked the car up there was alot of play in it. It just wasn't rolling freely.