What Kind of Fuel Am I?

Many people pick a gas station based on price and/or convenience, probably because they feel that most brands of fuel are pretty much alike. Is that really the case, though? Since your choice of gasoline directly affects engine performance, economy and longevity, along with the environment, it pays to know the facts about different types of fuel.

RON, MON
Let's start with some basic information about octane rating. The term octane is a familiar one and it's important to follow the recommendation that your vehicle's manufacturer specifies in the owner's manual. That number is actually an average of two different octane numbers (Motor Octane, or MON, and Research Octane, RON). These refer to the fuel's ability to resist "knock" (fuel igniting before the ignition spark and resulting in a "flame front") under different driving conditions. MON affects knock at high engine speeds or loads, RON at low ones.

Should you just go for the bargain du jour and pocket the difference?
What do these two types of octane measurements have to do with fuel selection? Well, if your engine knocks or pings under only one of these conditions, changing to a different brand of fuel may solve the problem. But if knock occurs under both conditions, then there's likely a mechanical problem that needs attention. It should not be necessary to use a fuel of a higher octane rating, unless all other remedies have been tried.

Fuel Myths
We should address a common myth about octane. Know that using fuel of an octane rating higher than that of the manufacturer's recommendation will not increase performance, fuel economy, lower exhaust emissions or increase engine/fuel system life. Usually, only vehicles with high performance engines (fitted with multi-valve heads and/or a supercharger) require a higher-octane fuel.


Does this fact about octane mean that "gasoline is gasoline" and they're all made according to the same formula? Should you just go for the bargain du jour and pocket the difference? Actually, oil companies are competing with each other, and that means that they don't share formulas with each other, nor do they necessarily get their crude oil from the same place. That means the final product will vary, from brand to brand and grade to grade. (And there's even some slight variation within brands, as gas is reformulated both seasonally and geographically, but that's another story.) You do truly get what you pay for—but is it worth it to you?

Price & Grade
Let's examine some of the differences in price and grade. The first is in the quality of the crude oil used. Cheaper grades have more undesirable compounds, such as sulfur, olefins, and diolefins, which cost the refiner to remove. The more of them removed, the higher the price. Since these compounds contribute to higher exhaust emissions, reduced service life of catalytic converters and sludge/varnish buildup in fuel delivery/intake systems, those who plan to keep their vehicles for many years would wisely opt for the better quality gasoline. One easy tip-off that marginal-quality gasoline is being used is a foul sulfur smell emanating from your exhaust (assuming there's no engine malfunction).