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Sure, individuals can test using whatever method they want to come up with whatever result they want, including driving for "free" on battery only, never running it out, and claiming infinite mpg.
However, it is the manufacturer (VW) that is claiming the 260mpg (or, more accurately, they are claiming 0.9l per 100km) and in order for them to claim that, I suspect (but have not confirmed) that they are basing the number on the current available European standardized FE testing, which would be NEDC. I don't know enough about NEDC to say if it considers the electric consumption of a PHEV, and so I can't comment how "valid" it is. But I do not believe the manufacturer would make the claim unless they could have it verified independently through standardized testing.
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