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Originally Posted by JCL
It is not a zero sum argument, and in my case, I don't think it is a cost issue at all. Measuring from fill to fill, over several tanks, my best mileage has always been on 89. YMMV, it depends completely on the quality of fuel you have access to. I use 91 when I travel, but at home, all my vehicles get 89. I do notice a decrease in performance with 87, and so I don't use regular, since it is obvious that the ECU is retarding the timing. That is not true with 89 pump octane, in my case. Again, YMMV.
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So there is chemically almost no difference between 89 and 91? I'm askin' not debatin'...and maybe I missed this, but the reason for 91 on the long runs is why? Fuel quality control or slightly more efficiency from a tank? I would ask, why not give a car the better stuff when it is working harder in stop & go city driving, no? Or, is 89 all the car requires? And what about sea-level driving vs higher altitude?