I occasionally have a lucid moment in which I notice something that had been staring me in the face for years. A few weeks ago, it occurred to me that the price difference between regular gas(87 octane) and super(91 octane) had seemed to stay the same (around 20 cents) even from the days when gas was a buck fifty a gallon. Since my teenage years, it had always been axiomatic that high test gas was only for rich folks that had fancy foreign cars and utterly foolish from a economic point of view. Then I got to thinking. At around $3.50 a gallon for regular and $3.75 for super, if I got only a one mile per gallon improvement in mileage on my family car, it would be cost effective.
Here's how it works at today's prices. I filled up at Sam's Club gas stand in Renton this morning. The 87 Octane was $2.91, the 92 Octane (they claim 92!) was $3.11. I get about 19 mpg city and 21 mpg highway in my Ford Ranger, let's call it 20 mpg avg. I only need to get a 7% improvement in mileage to pay for the extra expense. That would be a 1.4 mpg improvement. (I would have needed twice that if the price difference stayed the same when gas was around $1.50 per gallon.) My driving routine varies so much, it would be hard to detect that small of a difference. But, clearly, my long held prejudice against paying for high test gas is no longer valid.
A little research on the web, (this is one result) and I have come to the following epiphany: Use what your Owner's Manual recommends!
It comes down to the ECU. It will be set up for what the manufacturer recommends.
If you use a lower grade of gas, it will "de-tune" the engine to avoid knock, aka pre-ignition. Using the cheaper gas will almost certainly decrease performance.
If you use a higher grade of gas, it won't recognise the better fuel and change injection and timing accordingly. You probably wont get significantly better mileage with it.
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