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I 'learned' last yr when driving to texas from ca, that when I use gas below the 91 octane that is required for my 02 Q45, the " ign " function reports less than the optimal number. And the result is less mpg. It also corresponds to a higher 'load'.
On my trip this week, I tracked ing, load, hp, fia, fwt, mpg, mph, to get a better idea of the value in running 91 octane.
My conclusion is that IF your vehicle say to use 91, then your engine will perform at optimal conditions. However, IF your use less than 91, the ecm will 'retard' the optimal perameters. If you weren't running a scangauge or closely tracking your mpg, you probably would not notice the difference until you push the car.
For example: driving fully loaded, or diving thru inclines, or driving in 90+ degree w/ a/c on.
When I ran less than 91, the 'ing' function would not report more than 29. with 91 gas the 'ign' function reports 32-36
My gut feeling though is that if your car doesn't require it, then using it probably WONT show any improvement. But that is just my guess. If your car hits 31-36 with regular gas, the 'ign' fuction can't run at 40.
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