But doesnt super give increased MPG right across the rev range, so the extra cost of super would still be offset by the increased mpg, whatever the journey type?
Around here super is only 3 pence more expensive than normal (137.9 plays 140.9) which is 2.17% extra, so if you get 25mpg on normal, it would only have to go up to 25.5mpg on super for it to justify the price difference (plus the added benefits of extra torque, obviously)