well done on the mileage.

It has been my observation here in central MA that gas stations are crooks. I typically aim for 93 octane V power shell in my 07 S60 turbo, and as crazy as some would say, I can tell whether it is real 93 or watered down / straight up 87. I can tell this once I leave the parking lot and assess whether the amount of "perceived" performance/power is commensurate with the relative throttle input I apply. I usually fill up and drive until the tank is close to empty.

Height differences and relative humidity as well as air temp will affect mileage to some degree.

I've had times whereI got close to 28 in my car, although I am hovering around 24.8 - i do need a tuneup.

I will say however, in rare cases I got close to 28 in my (previous) 98 V70XC, with snow tires on it, driving to Maine. I anticipate perhaps the overall quality of the roads in NH and ME may have assisted....as you know, the roads in MA are terrible.

Best
Steve