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Recently, gas is much more expensive in the US but still lower than many countries. The rising costs led me to do some math.

Question: How can I mix gas and comply with the recommended 90 octane (AON) level without having to buy more than one grade at a time, i.e. alternate tankfull of different octanes? One can buy half a tank of 93 octane and half a tank of 87 octane if one doesn't mind the trouble.

Assumptions:
1. one always fills the tank when it is HALF full (SAAB doesn't recommend an almost empty tank because of oxidation resulting because of an exposed fuel pump)
2. 89 octane is 10c less per gallon than 93, 87 is 20c less

Results:
1. Fill with 93 octane alternating with 89 (93/89) = 5c less, long term octane range 90.33 - 91.67 (lowest octane will be 90.33 immediately after filling with 89 after several tanks)
2. Fill with 93 octane alternating with 87 (93/87) = 10c less, long term octane range 89.00 - 91.00 (violates 90 octane goal)
3. (using the above shorthand) 93/93/87 = 6.7c less, long term octane range 89.57 - 92.14
4. Filling with 93 more than 2 tankfuls then using 87 (93/93/87 or 93/93/93/87) will always result in slightly less than 90 octane immediately after filling with 87.

Notes:
1. If 91 octane is the maximum available, you would have to always use 91 in order to always have an octane of 90 or more. If you use 89 for alternate fillings, the average octane will be 90 but the minimum octane immediately after filling with 89 will be less than 90.
2. If 89 octane is less than 10c cheaper than 93, you might not consider the price low enough and might want to skip an alternating tank of it for the time.

QUESTIONS:
1. Noticed any difference in using 93 octane versus 90 (or 90.33 if one alternates 89 and 93)?
2. Are there any difference in concentration in detergents for different grades? If so, is the lower concentration sufficient?
3. Is Top Tier Gas advantageous? For a brand to receive the Top Tier qualification, all grades, even the lowest, must meet their standards. One of the requirements of the standard is that it must have 8-10% ethanol. In the past, some people avoided ethanol blends.
 

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Realize one thing first, Dave. There are only two types of gas buried in tanks under your gas station-- the lowest and highest grade. The mid-grades are all mixed by the pump. Since Florida has self-serve gas, you can mix whatever you like, provided you know the volume of empty space in your tank.

If your recommended minimum octane is 90, you may find it easiest to run 89, which seems to be mid-grade in a lot of places.

Your other ideas are fine, but you have to keep track of what you did the last time. I'm not that organized. Personally, I used to mix 93 and 89 with each refill to get 91 for one of my old cars. Yes, I had to pay twice, but swiping a credit card is pretty easy.
 
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