First, the dealer doesn't test for MPG. That is done by the Federal government, and is done according to a very sophisticated and convoluted testing requirement (commonly referred to the FTP, or Federal Testing Protocol).
The likelyhood of ANYone ever driving according to this procedure is VERY slim.
The reason for it is so that there is a platform that EVERY vehicle does exactly the same thing, and therefore there is a basis for real comparison.
The FTP is used for a whole host of data, but the primary focus is both emissions and fuel economy.
As fo the fuel grade, the ONLY difference between 87 octane and 92 octane has nothing to do with the energy stored in the fuel. It has ONLY to do with the anti-knock compounds.
Cars that require the premium (Porsche, Cadillac, BMW, etc) are designed for the more expensive fuel, and their ignition timing is programmed for the added anti-knock additives.
If you are driving a car that only requires 87 octane, using the more expensive 92 octane is merely a waste of money, and you will see no appreciable improvement in either economy or performance.