I have had a pressing question haunting my mind for quite some time, and I need an expert opinion. My car calls for premium fuel (2007 Honda Civic Si), yet states (in the owners manual) not to use gas below the octane level 87. Now I purchase my gas from Sinclair and the octane levels read: Premium: 91 Super: 88 Regular: 85 I don't have a problem using premium gas, other than the fact that the price is escalating rapidly. I would prefer to use a gas with a lower octane level, however I don't want to use a 88 octane gas. So, my question is: If I wanted to mix the 88 with the 91 and achieve a overall octane rating of 89, how would I do this? Parameters: 12 gallon tank I'm not highly aware of the properties and characteristics of mixing octane levels, but if I were to do the math I would do this: For every 1 gallon of 88 octane there is 88% iso-octane and 12% heptane. For every 1 gallon of 91 octane there is 91% iso-octane and 9% heptane. (8 gallons of 88 octane + 4 gallons of 91 octane)/12 gallons = 89% iso-octane and 11% heptane. Is this correct? Also, is there a large difference between 1%-2% iso-octane, relative to a car engine? Thanks.