Found this on a forum, its from EVOs golf gti test not my write up,
In the January issue of evo magazine, they ran a test comparing supermarket fuels against the top brands in the UK, to see which yields the greatest performance gains, if any. They actually perferomed two tests, one with 'regular' 95 octane petrol, and the other using the 'super' 99 octane. (In Europe, octane is measured in RON, where as over in America, 95 would be equivalent to maybe your 91 octane?)
For the tests, they used two cars. A V10 BMW M5 and the VW Golf GTI.
(We will ignore the results for the M5 as regardless of what fuel it was running, from the everyday supermarket 95, to the 99 Super Unleaded Shell Optimax premium fuel, they all produced the same performance figures. Does this mean rip is right? No of course not, the explanation they give was due to the BMW's advance engine manegment system. But since few of us drive state of the art V10 powered cars, I won't go into detail about the explanation they give as to why.)
So getting back to the Golf GTI, the results were as follows:
Shell 95
-Max power: 205bhp @ 5600rpm
-Max torque: 234lb ft @ 2650rpm
BP 95
-Max power: 204bhp @ 5600rpm
-Max torque: 242lb ft @ 2300-3100rpm
Supermarket 95
-results thrown out as fuel samples not consistent
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Shell Optimax 98
-Max power: 209bhp @ 5500rpm
-Max torque: 242lb ft @ 2500-2900rpm
Tesco 99
-Max power: 212bhp @ 5500rpm
-Max torque: 242lb ft @ 2400-3500rpm
BP Ulimate 98
-Max power: 212bhp @ 5300rpm
-Max torque: 252lb ft @ 2400-2500rpm