Many oil companies now offer "premium" petrols - in the UK, these include Shell V-Power (formerly Optimax) and BP Ultimate. People frequently ask if these fuels are worth the extra money that they cost - my answer is, "it depends".

The main advantage of V-Power etc is the higher octane rating - typically 98 RON as opposed to 95 RON for normal UK unleaded. The effect of this on your car ranges from nothing to maybe 10% more power & torque, depending on the characteristics of the engine. Certain engines - especially turbos - are "knock limited", that is to say, the ignition cannot be advanced to the optimum point because the fuel/air mixture explodes spontaneously causing engine damage. The higher octane rating allows more ignition advance and hence more torque. Providing the engine ECU has a knock sensor, it will automatically add this extra ignition advance whenever V-Power is used.

If your car manual quotes a recommended octane and says something along the lines of "lower octane fuel may be used but with reduced performance" you can be pretty sure it has a knock sensor. If your car manual specifically recommends use of 98 RON fuel, but you normally use 95 RON, then a premium fuel such as V-Power should give a noticeable performance boost. One common misunderstanding is that using a higher than intended octane fuel is actually bad for performance and economy, because it "is too hard to burn" or "burns too slowly". In fact this is wrong; unless the ignition system is extemely marginal, using high-octane fuel in an engine designed for low octane should not cause any problems.

In terms of fuel economy, being able to advance the spark at full load certainly gives better economy, since the engine is making more efficient use of the energy in the fuel. It also potentially allows use of a higher gear in certain situations (due to the increased torque), which is in itself more economical. Finally, many engines run very rich at full load in order to cool down the exhaust gas, and if the ignition is retarded due to knock, even more additional fuel is needed. So under high load conditions, higher octane is definitely good for economy.

However, knock isn't an issue at part load, where the engine spends most of its time. In terms of overall "real world" fuel economy benefit due to increased octane rating, the effect is likely to be small unless you drive much of the time at high load - either because you have an "enthusiastic" driving style, or because you have a relatively large, heavy car with a relatively small but powerful (typically turbocharged) engine. (The latter is becoming increasingly important with the move to downsizing.)

An additional advantage is that, due to their different composition, "premium" fuels are typically two or three percent more dense than "normal" fuel, which means that each litre contains two or three percent more mass of fuel, and hence two or three percent more energy. All other things being equal, that automatically translates into a two or three percent improvement in fuel consumption (miles/gallon or litres/100 km).

Finally, "premium" fuels usually contain more effective detergent additives to keep the engine internals (injectors, valves, combustion chamber, etc) clean. In general I am sceptical of the need for anything other than a good quality "ordinary" fuel to perform this service, but if I did want additional cleaning I would more inclined to run my car on a "premium" fuel for a month or so than to spend the same money on an aftermarket cleaning product. (There are some claims on the Net of Optimax damaging engines, for example by washing deposits into places where they will do harm, but personally I am sceptical. Shell have suggested that this might be an "urban myth" related to the genuine problems with Formula Shell in the 1980's; I can't offer any evidence either way.)

So, premium petrols are great and everyone should use them? Well, not quite. All engines benefit from the increased fuel density, and the cleaning additives. But the benefit from the higher octane rating is small on those engines intended to run on low octane fuel, and zero on those without knock sensors (for example Rover's 4-cylinder K-series). (Some engines are naturally not "knock limited" - the optimum ignition advance can be used at all times without knock occuring - and others are only slightly limited.) In keeping with the sceptical outlook of this website, I do feel that the marketing for such fuels, by the use of wording such as "up to", can give a misleading impression of the average improvements to be expected - and the Advertising Standards Authority agrees.

Personally, my view is this:

• if your manual specifically recommends use of high octane fuel, and particularly if you have a small turbo engine in a large vehicle, "premium" petrol should be used whenever possible
• if your manual recommends "ordinary" fuel but your engine has knock sensors, use of "premium" petrol on occasion may be worthwhile
• otherwise, it is hard to justify unless you particularly feel your engine needs additional cleaning (though in the US, I would be very tempted to stick to "Top Tier" gasoline to be on the safe side)

The UK Fifth Gear TV program recently ran a story on "premium" fuels, including some test data that largely agrees with the comments above. BP also now only claim a typical fuel economy benefit of around 3 - 4% from their premium fuel, which seems plausible.

I am not enough of an expert on diesels to say whether or not the extra cost of a "premium" diesel fuel is justified.

As an aside, some commentators suggest that different petrols can give wildly different fuel consumption (gas mileage), maybe as much as 50% between the best and worst. I find this exceptionally unlikely - the critical parameters such as density, energy content, boiling curve, etc are pretty closely controlled in all highly developed countries, and so unless you get a "defective" batch I just cannot see how there can be this sort of spread (5 percent, perhaps, but never 50%). To my mind, it is far more likely that the natural variability in fuel economy is responsible for the observed variations.

After all, if there really were such a variation (even in fuels that cost approximately the same), why would the oil companies not either:

a) always make the best possible fuel, and advertise its economy benefit (thus greatly increasing sales), or

b) always make the worst possible fuel, so (as the conspiracy theorists would have us believe) increasing oil consumption,

rather than simply delivering the "random" variation supposedly seen.

What is however probably true is that the additives added to the "base" fuel can and do vary greatly between different brands. While poor quality additives are highly unlikely to affect economy in a single tankful, or even a dozen, long-term use of a fuel with insufficient detergency will eventually lead to clogged injectors and possibly "gumming up" of other intake system components as well. All fuels sold in highly developed countries should meet the minimum standards, but there is some anecdotal evidence that they do not.

The question of the effect of alcohol (Ethanol) in petrol (gasoline) is also considered in the conspiracy page.

The Gasoline FAQ is an excellent, if heavy-going, reference to petrol and covers many of the same topics as this site.

Footnote: RON, MON and Octane ratings

The Octane rating of a fuel is assessed by burning it in a test engine and measuring the resistance to knock. Two different methods are used, known as "Research" and "Motor" - they are similar in principle, but involve different loads, speeds and temperatures. These tests produce the Research Octane Number (RON) and Motor Octane Number (MON). Typically, the MON is about 8 - 10 points lower than the RON.

Critically, in Europe the "Octane rating" of fuel is almost always the RON, while in the US it is (RON + MON) / 2. So the same fuel would have an octane rating of 95 in Europe and 90 or 91 in the US.