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So you're saying using lower octane gas in my Porsche lowers my mileage. But you don't want to say increasing the octane back up causes the opposite effect. I'm not sure what you think happens to my mileage when I move from mid-grade to premium. The mpg on my car computer moves in the positive direction about 3 mpg.
It's like saying you get lower when you walk downhill, but not higher when you walk uphill. You must live in a different universe than I do.
The reason "we" don't all get a benefit from increasing the octane is that most cars lack the sophisticated computer control system to retune the engine on the fly. As I've been saying from the beginning, "It depends upon the car."
You are correct that higher octane will increase gas mileage in engines that are capable of adjusting their ignition timing. The reason why they might recommend a lower octane rating, despite the decrease in gas mileage, is that running with a higher compression ratio puts more wear and tear on the valve springs and other engine parts, so it is a balance between the cost of fuel, performance, and the cost of engine maintenance.
You are correct that higher octane will increase gas mileage in engines that are capable of adjusting their ignition timing. The reason why they might recommend a lower octane rating, despite the decrease in gas mileage, is that running with a higher compression ratio puts more wear and tear on the valve springs and other engine parts, so it is a balance between the cost of fuel, performance, and the cost of engine maintenance.
I don't think there is more wear & tear on valve springs, but higher compression engines will likely have higher maintenance requirements, all other things equal. The computer detuning is to prevent engine damage if you end up in a location where premium gas isn't available.
I JUST BOUGHT A 300C WITH HEMI ENGINE. User manual calls for using 89 octane. 10 cents more than regular. Is it necessary?
You're probably best off finding a 300C Hemi forum to hear from user experience. As for me, we run 87 octane in my wife's 07 Lexus Rx400h though the manual recommends 91 octane. My rationale is the the V6 engine used in the hybrid is a detuned version and has a lower compression ratio. No difference in fuel economy and no noticeable difference in performance…granted it's a V6 hybrid with a lot of low-end torque.
I just wanted to chime in on this thread to provide a useful data point. A couple years ago I owned a VW CC. It recommended premium gas, and got about 30mpg on the highway. One day I tried regular gas at a fill-up, and I immediately noticed that I was getting 32mpg highway. Nothing different- totally routine driving, same temperature, etc. I tried it again- got a tank of premium and went back down to 30mpg then got a tank of regular and back up to 32mpg.
Since I was leasing and didn't care about longevity, I didn't even care that the sticker said "premium recommended"; I just ignored it and used regular for the rest of the time that I owned the car. No reason to flush money and gas down the toilet.
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