I used to test a lot with my old car 96 Benz C-Class, different brand, different grade of petrol. I found using super the car \"feel\" to have more low-end torque, so you don\'t need to step on the accelerator as much with the same acceleration, therefore use less fuel.
BUT, it really depends on the \"mixture\" of. different petrol ststion. Some station petrol have strong \"feel\" but use up very quickly, some ok feel but can last very long time.
I keep all my refueling record and each time I fully top up with petrol so can get accurate record.
My car now run on Super Unleaded running at around $1.6 ~ $1.7 after discount / 70% Highway, 08 Stepwgn 2.4. The difference on 1 full tank 50L can differ from 450km ~ 550km from full tank to refueling warning light on.