JHarris, I think you should buy your gas in Indiana. The RVP 7.8 stuff in Indiana is a heavier-molecular-weight gas that doesn't evaporate as quickly as conventional gas does (reduces smog from gas vapors). The ethanol-oxygenated stuff in Kentucky will give you lower mileage. Buy a tank of each and see what your results are. I'd be curious to know if there's any difference between the RVP 7.8 stuff and the fully-conventional gas you can get one county out.
Wikipedia has a decent write-up on gasoline and it's many variations. Here's some California-specific info
Coastie, I thought you were in FL! If you're in SF driving on 5.7% ethanol Cali-gas, I cannot believe that you are getting 18+ mpg. I don't know what's going on with your calculations, and I don't doubt that you are getting better mileage with correct plugs, but 18 mpg doesn't add up.
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