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Originally Posted by lance-n
I ran premium for the first 11 months and 9,200 miles. Diligently checked mileage at each fill-up and made between 13 and 18 mpg, depending upon what conditions I was driving. The 18 was almost all on the 1,000 mile trip to Cabo. Since then, I have a short daily commute here and do quite a bit of beach and sand driving and am certain that is the reason for my somewhat lower mileage than most others have experienced.
After reading the many posts regarding the no risk of switching to regular unleaded, I grudgingly decided to test it out. I have had bad experience with regular, as my 1995 Bronco shows way worse performance using it. It performs with lower mileage and way lower power using regular unleaded.
However, the H3 shows the same mileage using regular unleaded or premium - 14.5 on the first tank and 15.1 on the second tank with no noticeable power difference.
In the other posts on this forum it is discussed (by some knowledgeable people) that the timing in the H3 engine automatically adjusts and compensates when lower octane fuel is used. So, I didn't want to believe it, but it is really true and now I save 10-15 bucks per fill-up.
If you sift through the BS (and all the fun), there are some extremely helpful nuggets of wisdom to be found on this site.
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The H3 has two spark timing tables - high octane and low octane. The data in 2D tables are arranged by RPM and cylinder airmass. The PCM starts on the high octane table and if knock is detected reduces the timing until knock is gone. The working timing table usually falls in between the high octane and low octane tables. However, the H3 is the only vehicle that I have seen that primarily stays on the high octane table which means there is no room for timing advance to take advange of higher octane gas. This is why you don't feel a power/MPG improvement with the higher octane gas.