There are basically only two grades of automotive gasoline that are refined - a so-called low octane and a so-called high octane. What is offered at the pump depends on the blending process and the Ethanol content. There are also specialty racing unleaded gasolines, but those are a completely group of fuels.
For the nine "west coast" states there are 87 and 91 pump grades. For Colorado which is mostly above 5000 ft there is also a 85 octane pump grade. For most of the rest of the country there is 87 and either 92 or 93 octane pump grades - depending on the refinery/brand. Before the Great Recession there was also some 94 octane super premium pump gas back east, mostly at Sunoco stations - but it only came from one specific refinery.
Now all the other mid level octane pump grades are simply made by blending at the pump itself. 89 octane is two (2) parts 87 and one (1) part 93. While 91 octane is one (1) part 87 and two (2) parts 93. If you want to do the math, you will find that the pricing of mid level 89 or 91 may not reflex its blending proportions, so buyer be where.
Because ethanol can't be pumped though the carbon steel gasoline pipeline infrastructure, it has to be delivered to and blended at the fuel depots just before being loaded into stainless steel tankers for delivery to the gasoline stations. Also 10% Ethanol content has an impact on the octane number of the pump gas. For example, blend 10% straight ethanol (which has an octane rating of 112 - laboratory grade Ethanol that has been chemically treated to remove all the water content supposedly has an octane rating of 114) with 90% say 91 octane pure gasoline and you get 93 octane pump gas. If you take 91 octane blended pump gas and remove the 10% ethanol content, then your get 89 octane pure gas, which is what is sold at marinas and at gas stations that offer "Ethanol free gasoline" for use in say your yard and power equipment.
Its all a numbers and blending game. As always YMMV.
