Thursday, July 27, 2006

Dear Friends:

If you put higher octane gas in your tank, that means you'll get better gas mileage, right?

Wrong.

If your car is designed to run on regular gasoline, spending extra money to buy premium does not make sense.

What are Octane Ratings?

Your engine burns a mixture of air and fuel that is injected into each engine cylinder, where a spark ignites it. Octane ratings measure the amount of pressure, or compression, the fuel can withstand before it ignites prematurely.

Regular unleaded fuel has an 87-octane rating. It will ignite at a lower pressure than 89-octane midgrade gasoline, or 91-octane premium.  If you use an octane level that's different than what the manufacturer recommends, there may be problems but they won't necessarily be gas mileage.

What is Engine Knock?

When fuel ignites prematurely, your engine may make a "pinging" or "knocking" sound. This is more likely if you put 87-octane regular in a high-performance engine, which boosts horsepower by igniting fuel at a higher compression level. These cars are designed to run on mid-range or premium gasoline.

Engine knock isn't always caused by the fuel, so it shouldn't be ignored. It may be a sign of problems with the electronic control systems, ignition timing or exhaust gas recirculation.

Can I switch from Premium to Regular?

Check your vehicle owner's manual. If your manual does not specifically recommend high octane you probably can switch. If the vehicle knocks, pings or runs poorly, try mid-grade before returning to high-octane fuel.

If you think you are not getting the octane rating you paid for, please contact the Arizona Department of Weights and Measures by calling (602) 255-5211 or filing a complaint online at http://www.azdwm.gov/complaints.htm.

As always, I encourage you to visit my Web page devoted to gasoline information at www.azgovernor.gov/fuelfacts. Please check the site often throughout the summer as we add fuel saving tips.

Yours very truly,

Janet Napolitano
Governor