Working in the petroleum industry, I feel a need to set the record straight as to how most of this works. (Please keep in mind, I deal with the state of Oregon, and sometimes Washington) First, the wonderful (not) 89 octane gas is indeed a mixture of both 87 and 91. Usually it is one of two mixtures, either 90/10 (90% 87 and 10% 91, or 60/40). This mixture is done at the station, usually at the pump. Why would you mix this when filling a truck and waiste space, when you still have to carry the original ingredients to begin with. Why not carry more of the original ingredients and mix at the station? (It’s all about saving money) If you really want to see it yourself, next time you fill up, take a look around the top of the pump. You should see a couple of odometers (just like in your car that keep track of the milage), if your getting more than a couple of gallons, you’ll see one click over more than the other. Certain cars do run better on higher octain than other cars. Really it just depends on your car, and there are just to many variables to go into. As far as your second question, how many storage tanks do stations have, it depends on what they sell, and the volume of the station. If a store only does 2,000 gallons a day in gas, chances are, they are only going to have two tanks (1 for 87 and 1 for 91, providing they don’t sell diesel). If this same station does sell diesel, then they would have three tanks. If it’s a higher volume station they may have more than one tank for one product (two tanks for 87 and one tank for 91). Take a look the next time you pull in to get gas or diesel, you can usually locate the tanks by a metal “cap” that slightly protrudes from the ground. I would not advise you go and “inspect” these, for someone might think you are attempting to do something devious. I hope this answers your questions.