Rick's Free Auto Repair Advice

Gas mileage and octane rating

Gas mileage and octane rating

Every once in a while I read a post on an auto forum where someone claims to get better MPG and performance when running a higher octane fuel as if gas mileage and octane rating are related. Or, they claim to get better gas mileage with one brand over another. Different grades of gas contain nearly the same amount of energy (BTUs). The ONLY difference between 87 octane and 93 octane is fuel’s ability to resist detonation and knock due to the heat of compression or ignition from carbon deposits in the combustion chamber. That’s it. Premium gas doesn’t contain more power.

The only exception to that statement is if you own a late model car equipped with a knock sensor and your engine already has carbon deposits in the combustion chamber. In that case, if the engine has been experiencing detonation from the carbon deposits and the knock sensor has been reporting the knocking to the engine control computer, it may have retarded spark timing to reduce the knock. If you then add a higher octane fuel, the computer will slowly readjust spark timing to get more performance and you’ll feel the increase. But it’s not coming from the fuel, you’re getting more power because spark timing is back to an optimal level and the higher octane is preventing knock.

If the manufacturer of your high compression engine recommends 93 octane and you fill it with 87, you’re going to get lower gas mileage. You may not hear pinging, but that’s only because the computer is a step ahead of you. The computer senses detonation by way of the knock sensor and retards the spark timing to prevent a double flame front (one started by the detonation or pre-ignition and a second one from the spark plug). Retarding the timing DOES lower your gas mileage. That’s why it’s never a good idea to run lower octane in a high compression engine.

Here’s another myth–different brand of gas give dramatically different MPGs. There are slight differences in gas between brands because gas is a blend of almost 150 different components and each brand uses their own recipe. But the differences are minor–at best 1 MPG. In many urban areas fuel is delivered by pipeline to fuel terminals. All the fuel from that particular terminal is the same. It’s only when the individual trucks from each brand fill up that the additives are poured in. Yes, the additives are different from one brand to the next. But as to the “power” contained in each gallon–it’s all the same. Gas is gas.

If your car starts better on BP than Shell, it’s because the additives work better in your engine. But you can’t make the claim that because it works better in your engine, it will work better in every engine.
© 2012 Rick Muscoplat

Posted on by Rick Muscoplat



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