Featured Articles

Unleaded Premium Fuel Is A Scam: Fact Or Fiction?

Posted in auto industry, Car Buying, Car Care, Engines, Featured, Fuel, General, Tips by Kurt Ernst | April 7th, 2010 | 12 Responses |

Is saving $6.00 per tank worth the risk of a blown motor?

A friend of mine (let’s call him “Stan”) is one of the smartest guys I know. He just authored a book on business process and works for one of the big financial powerhouses as some sort of a training guru. He recently hit me up for a recommendation on a domestic sedan, so I countered with the usual list of questions. One of them was “are you willing to use premium unleaded fuel”, to which Stan replied “unleaded premium is a scam”.

When I stopped screaming at my email, I realized that if Stan thought this was the case, then so do a lot of other people. Manufacturer’s don’t do a good job of explaining this, and dealers are reluctant to point out that your new car will cost you more money to operate than you thought. So what’s the real deal? Is unleaded premium a requirement, a luxury or a scam? If you need it, why do you need it?

Uh oh: pistons aren't supposed to look like this one.

First, let’s start with a little science. A fuel’s octane rating indicates how combustible it really is; contrary to popular belief, low octane fuels (standard unleaded) are more combustible than high octane fuels; specifically, they have a lower flashpoint. High octane fuels have a higher flashpoint because they’re intended for use in high compression engines. Oh, crap, I’ve lost you, haven’t I?

Let’s back up one step and think about how a four stroke motor works. When the piston starts up the cylinder in the compression stage, both the pressure and the temperature inside that cylinder rise. In a diesel motor, this compression alone is enough to create the ignition; in a gasoline motor, a spark is usually required. Why do I say usually? Because using the wrong, low octane fuel can cause the air / fuel mixture to ignite before it’s supposed to. This is called detonation, and bad things happen to motors with uncontrolled detonation. Things that require motors to be replaced or rebuilt.

Connecting rods aren't supposed to look like this, either.

Here’s the good news: modern technology has a way to address this. A device called a “knock sensor”, usually positioned on the side of the engine block, listens for the noises associated with detonation. When it senses them, it signals the motor to retard the timing, which allows the combustion to occur when it’s supposed to. The down side is that when this happens, you motor is making less power than the manufacturer intended it to. It’s literally trying to keep itself from blowing up.

Now some bad news: knock sensors can’t always correct ignition timing enough to compensate for the wrong octane fuel. The editor of an automotive website recently found this out the hard way, when the brand-new Cadillac SRX Turbo he was driving blew it’s motor climbing a mountain pass outside of Ventura, California. The editor admitted to mistakenly filling the press fleet vehicle with regular unleaded instead of the required premium; the sequence of events that followed (retarded spark, turbo boost, cylinder overpressure, cracked engine block) left him stranded by the side of the road. Per Cadillac engineers, use of the wrong fuel was a contributing factor and not the primary cause; still, the incident may not have happened had the editor used the grade of fuel specified by the manufacturer.

Confused? Don’t be, because here’s the breakdown: if your owner’s manual says unleaded premium only, use it every time you tank up. Sure, you’ll pay a few dollars more each time you fill up, but I’ll bet that it’ll still be less expensive over the life of the car than buying a new motor. If you want to see what fuel is required BEFORE you buy a car, just check inside the gas cap lid. If it says, “premium fuel only”, that’s what you need to use.

If your manual says “premium fuel recommended” you can probably get by using the mid-grade stuff for normal driving. If you’re towing a trailer or driving in extreme conditions (up mountain passes, across the desert in summer, etc.), then buck up for a tank of premium. Don’t use the cheap stuff, no matter how tempting it may be.

So what if your manual just calls for the use of “unleaded fuel”? Can you get better performance by running premium unleaded? Is the good stuff formulated to run cleaner, remove harmful deposits, whiten your teeth and leave your laundry smelling spring fresh? In a word, no; running premium fuel in cars that don’t require it will do nothing other than drain your wallet more quickly.

Our Best Articles

Leave a Reply

Your email address will not be published. Required fields are marked *

12 Responses

  1. Dave says:

    Here comes the pitch…uh oh, it’s another gas myth\urban legend. But wait!!! What’s that that Kurt has in his hands? It’s the Owners Manual. He’s pulling the old “RTFM” move.

    He connects…

    …and that pitch is outta there!!! Another Home Run for RL!

    Sorry, a crappy baseball metaphor was all I could come up with. Leave me alone, I’m tired. ;-)

  2. Kurt says:


  3. Joel says:

    Articles like this one make this site a real value. Thanks yet again, Kurt

  4. Kurt says:

    Thanks Joel!

  5. Greg says:

    Thanks for explaining what I try to so dearly to non-car lovers!

  6. JW says:

    Thanks Kurt. You are one of the few (other than myself! lol) that knows WTF they are talking about. Keep this guy Ride Lust! And your analysis is spot on brother. As far as premium in unleaded car, I would imagine it would be bad for it as well. Because it has a higher flash point I would think it wouldn’t burn as well and make more carbon deposit and such. That last point is pure speculation though, nothing to back it up. My Beemer requires the “good stuff” but I use plus normally like you say and it runs fine. But I didn’t realize the “knock sensor” is just really a glorified microphone. I thought it was electronic mojo working.

  7. Mark says:

    Only one small gripe. I own an original Honda Insight and a Toyota Prius. I can tell you flat out that in both of these cars, I get about 10% better gas mileage running Mid Grade over Rot Gut unleaded. On the few occasions I’ve put premium in them, I can get another 5%.

    Is the extra mileage worth the extra cost? Not really for me and I usually stick with the standard unleaded. However, for long trips, it makes a difference.

  8. Michael says:

    very good article Kurt. I can appreciate the facts being told.

  9. Humanparody says:

    I always use the cheap stuff in my ’87 Chrysler Fifth Avenue. It says right in the owner’s manual that 91 octane is recommended, but 87 octane (ironic much?) will suffice.

    One of these days when I get a job, I gotta fill up with the Screaming Eagle of fuel instead of the Bud Light, just to see what sort of difference it makes.

    • Kurt Ernst says:

      Humanparody, even 103 octane race gas won’t make much of a difference in a 24 year old Chrysler. Spend your hard-earned money elsewhere.

  10. clive says:

    I have a 1976 AMC RAMBLER MATADOR 360 V8 which has been rebuilt and i was wondering whether to use premium unleaded in this vehicle.it is not a daily driver.so the question is which brand of premium unleaded and why?will it damage the engine more than std unleaded?will i get better performance,KPL(kilometers per litre)economy.

    • Kurt Ernst says:

      Clive, without knowing specifics, I really can’t give you sound advice. Here are a few tips:

      1) When the engine was rebuilt, did you increase the compression ratio to boost horsepower? If you did, then premium unleaded should be used,

      2) Did the owner’s manual originally call for premium unleaded? If so, you should use the higher octane stuff.

      3) Does your engine knock or ping under load? Try accelerating up a hill in a taller gear, then listen for pinging. If you hear it, you need to run a higher octane fuel.

      I can’t give advice on brands of gas in Australia, but as a general rule I use only the name brand stuff over here. Good luck, and I hope this helps!