I live in California and the best octane we can get here is 91. I have an 03 Z and notice a pretty good difference in power when the weather is cooler, and living in inland Southern Cal, it's going to soon be between 90-100 degrees most days this summer. I can buy Sunoco 104 unleaded racing fuel for about $7.33/gal. incl. tax, which is spendy. I calculated that if I mix 2.5 gallons of 104 with 15.5 gallons of 91, I get 92.8 octane, but this comes at about an extra $20 each time I fill up. Will I notice an increase in power, or should I say, not force the timing to retard on hard acceleration by jumping from 91 to 92.8 octane? Two gallons gives me 92.4, three gallons gives me 93.1, four gives me 93.8, and five gives me 94.6. What octance do you think GM set optimum power for?