91, 95, 95e or 98 RON...:-( Help )

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Firstly I'm referring to WP and WQ Fiestas, may not be right for newer models. Fuel grade is all about 'pinging'. Without getting too technical, the Fiesta has a slightly higher compression ratio than many engines, 11:0 opposed to many 10:1 for similar cars. Higher grade RON 95/98 is therefore a better option to reduce ping in these engines specifically. Having said that modern day cars have electronics and what not that means it will run on RON 91 petrol just fine. Personally I have experimented with all 3 grades. The premium grades are definitely noticeably quiter and smoother. In terms of price wise, you definitely get a reduction in fuel consumption but the price is obviously higher. The math works out that it comes at about a 5-10% premium ie. if RON 91 cost you $10 per 100km than RON 95/98 cost you about $10.50-$11 per 100km. In terms of performance it is too hard to tell if it's a real power gain or just placebo effect. Personally I stick with RON 95/98 because I would believe its better for the engine and the final price difference is so small.