I've logged every fill up in all my cars. Best I've ever had is 56 (calculated tank fill-to-fill). This is under perfect conditions - 15 mile commute, country roads, steady 40-45mph, summer).
I reckon, taking 55 as a realistic best, make the following subtractions:
-5: motorway 70mph
-5: winter
-10: cold engine (first couple of miles of a journey)
-10: hard urban (though a good driver will do a lot better)
-5: aircon (summer), -1 aircon (winter)
-5: maintenance (sticking brakes, tyre pressures, tracking etc)
At the moment I'm getting 45mpg, on a 20-mile motorway commuter in winter. A relative who does all short cold urban gets 35 according to the display.
Nearly all my fuel is Shell. I have used VPower, no change in mpg, though does run more smoothly. Very subjective this, but I always think BP gets a lower mpg, in all my cars - plus its the most expensive. Since it's all the same fuel feedstock, but different additive concoctions per brand, then what does that tell you? (Note VPower and Ultimate are made differently, they have a different feedstock (I work in an oil refinery)).