I think some of the replies here were a little dismissive. Super unleaded is there for high(er) performance and it just depends on whether your particular car can take advantage of (or be aware of) the higher octane.
Now, my old Astra 2.0 GTE just loved high octane fuel - it was fast anyway but putting in the old super made it recalibrate its computer for the first 10 miles after fill-up (there was light to show it was doing it) and it went even faster once it had settled down. Ditto when filling up again on regular octane stuff, except it went a bit slower.
Trying the same trick in my wife's Suzuki Alto Auto 1.1 16v (don't snigger at the back) had absolutely no effect whatsoever. I'm guessing that the ECU just never tried adjusting for different fuel quality; after all, it's just not the sort of car that owners would want to 'supe' up.
Coming to my own GP 1.3 mjet (diesel) I can absolutely say that Shell's V-power has a very definite effect on its general driveabily - and it's not a placebo effect either. Although the 1.3 is already a very impressive engine (can easily out run any 1.6 petrol car, eg Vectra) the Shell V-power just makes it more responsive generally (turbo lag notwithstanding).
My previous standard Astra 1.6 petrol 8v also was much smoother running BP Ultimate rather than 'regular' fuel.
So it just goes to show that it is the engine management system that really determines whether the 'super' fuel is going to make a difference. The physics is simple: a higher octane fuel will burn faster thus giving more power or more mpg depending on how you use that extra oomph!
I'm very surprised that a 1.4 petrol sporting should only get 28mpg though; and I was worried about my 50mpg from the magnificent 1.3! I think I'll say not another word about poor mpg from my diesel!
Oh yes - follow the advice for staying away from Tesco or other supermarket fuel. It's cheap for a reason!
