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posted: 1/20/2013 4:17 AM

Buy correct fuel for optimal performance

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Q. I enjoy your column in the Daily Herald every Sunday. Here is my question.

I recently bought a 2000 S-Type Jaguar from the original owner. It only had 53,000 miles on it and I got it at a good price. I knew parts and repairs would be more expensive but I figured the savings over buying a new or used late model car would help pay for the more expensive potential repairs.

My question is related to octane. The owner's manual says to use a minimum of 91 octane gasoline. The top gasoline at most stations is 93 octane, and 89 octane is also available. Would it hurt anything if I filled up with 93 octane and top it off with the 89 octane when I got down to a half tank, for a 91 octane average? I would then repeat this practice.

A. Thanks for a great question!

You could do what you suggested but the chances of you getting the blend right are not in your favor. This is a question I get a fair amount from folks who's car requires premium fuel. Do I have to put premium in, because it seems to run all right on regular?

Today's cars adjust to the octane you put in the tank. If you have a car that should be running on premium gas and you put in regular, the car will adjust in such a way that you will lose performance and fuel economy. If you have a car that only requires regular fuel, it will not benefit you to buy a higher octane fuel.

In my opinion, using a lower octane than recommended is an offset that's not worth doing because the car will not be running optimally.

What you do want to be careful of are the new fuels coming out that have more than 10 percent ethanol. Unless you are driving a flex fuel vehicle, don't use anything above 10 percent ethanol as you could damage the fuel system or engine and it could invalidate your warranty. At a minimum you will probably trigger a check engine light and the car will run poorly. It also can be difficult for a technician to diagnose unless he has trained himself to look for this.

You indicated the manual for the Jag says to use 91 octane as a minimum. That tells me the preferred is probably the 93 octane. My suggestion for your Jag is to buy the 93 octane and enjoy the ride!

• Douglas Automotive is at 312 S. Hager Ave., Barrington, (847) 381-0454, and 123 Virginia Road, Crystal Lake, (815) 356-0440. For information, visit Send questions to

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