I have the 20" wheels and have had the car just under 5 weeks now (less 1 week for PPF/ceramic) and i have over 4100km on it now (just over 2500 miles).
Took the family from Windsor ON (Canada) down to Knoxville TN and back and i was really impressed on the mileage, I think 400 miles per tank is entirely reasonable as I am now getting between high 500km up to 700 km per tank. Its a multivariate equation - primarily being that the mileage will improve as drivetrain break-in occurs but my foot is also getting heavier! The above Knoxville trip was after break-in service and i got around 24 mpg (imperial) which i think is amazing considering i had my wife, 2 teens, and 300 lbs of luggage and was clipping along -
averaged 132km/hr (82 mph) for the round-trip and that includes the traffic crawling construction slow downs and rush hour... suffice it to say velocity was made up in other areas to get that average
Our 2011 buick enclave would have used the same or more fuel going only the speed limit - and no massage!
PS> I kept it in efficient / D1 most of the way to keep revs down - it won't hold the gear as long and will upshift to overdrive sooner which helps on the highway.