I am now getting 20 in City with Costco Premium gas in Phoenix for over a month, yes in stop an go conditions not highway. :D :D :D
There is no way you can get that big of a difference in mileage lol.
It does sound like a big leap. But if the factory specs call for 91, then it's possible you could see a drop in mileage using 87.
In order to get a good comparison test, you'd have to carefully monitor the factors that can change A/F, gasoline/engine performance and use more than just a couple of tanks of fuel....
Was the gas used from the same brand and gas station?
Was the test route driven exactly the same with the same MPH under the same driving conditions?
Was the ambient air temp, RH, and barometric pressure the same?
Did you measure and start with the exact same amount of gas in your tank?
believe it or not proper tire inflation does increase mpg...