2006 XC70 and E15

[MoVolvos]

ECM detected subpar octane number and retarded the ignition and cut boost in half to save the engine. Your car was not able to burn as much fuel as it usually does with 91-93 octane gas. It was accelerating slower and used less gas.

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Can you provide a link or two or testing to that analysis or conclusion?
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I drive with this thing: https://github.com/vtl/volvo-ddd and use the highest grade gas available. With the stage 1 tune my XC70 has I see boost pressures of 11-12 PSI easily.

When traveling up up up north in wild parts of Quebec, where they only have regular gas, I once tried to pass a semi on James Bay Road, only to find XC70 is gutless and barely able to pass the semi. I looked at the boost number, it was no more than 5 PSI. BTDC was also retarded.

Filled with 93 when it was available, yet it took the engine half the tank to recover from 87.

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Make sense that the ECM would detune the engine of a fuel specific Stage 1 tune due to the lower octane, although it does seem extreme.

As for the OP his increase in mileage is from better cleaner combustion or more complete combustion compared with the 91-93. It is a high mileage motor so the 91-93 may not be fully utilized or benefited compared with a car with a less mileage engine.
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[Krons]

Filled with 93 when it was available, yet it took the engine half the tank to recover from 87.

I’ve ran E85 a half dozen or so times in my Flex fuel Nissan Frontier when it got cheap and noticed the same…the 30% fuel economy hit took about 1/2 way through the next tank of E10/15 to recover. Generally E85 isn’t more than 30% less than E10/15 so the truck typically gets E15.