In general, gas stations keep stock of only two grades of gasoline; the highest and lowest octane available. Intermediate grades are produced on-demand at the pump, by mixing the higher and lower octane fuels together at a specific ratio as they are dispensed.
In the case of this pump (located at a Hess station in Fishkill, NY), we can see that the vendor is stocking 87 and 93 octane, and blending them at a ratio of approximately 67% / 33% to achieve their mid-grade 89 octane.
Given this, we would expect the price of the 89 octane to reflect a 67/33 ratio of the prices of the 87 and 93 octane fuels, and yet it does not. The price of the 89 is exactly halfway between the two extremes, which we would call "fair" only if the mid-grade fuel was a 50/50 blend of the two other fuels; that would make it 90 octane instead of 89.
Seeing this sort of thing bothers that small part of my brain which processes OCD-type activity.
Was not aware of the blending, thought it was a separate tank.
In some older gas stations, they do have a third tank. Often, this dates back to when leaded gasoline and/or kerosene were sold at the station, and so the tank space is available. The refineries, of course, still only make two grades; in the case of three-tank stations, the truck simply dumps the correct amounts of the two fuels into the "mid-grade" tank when it does the fillup.
Modern fuel pumps, however, are pretty much all capable of blending fuel to any desired ratio, so for newer stations (those built since, say, the early 80s), running a two-tank system is beneficial in nearly every way- lower construction cost, fewer EPA permits, and faster refilling from the truck with less chance of error.
In fact, some fuel stations offer an absurdly large number of octane choices, such as this one:
They're certainly not hosting five tanks at that station. Just presenting the customer with way too many choices.
When my neighbor was a salesman, he got in trouble from his company for buying mid-grade gas on the company card, even though it was 3 or 4 cents a gallon cheaper than the low-grade due to some additive (probably ethanol).
Does anyone even buy mid-grade gas? I never have, and I don't see why I ever would.
In Iowa, midgrade is usually cheapest by $.10-.20.
When you say that "midgrade is usually cheapest by $.10-.20," are you saying that it's cheaper than whatever the lowest grade is?
Originally Posted by Enginerd
I never have noticed any performance benefit from 89 over 87...but with my daily driver, using 93/94 octane on a 10 gallon fill up, 10*.40= $4.00 is roughly equal to the 2-3mpg hwy *10 gallons= 20-30 miles ~ 1 gallon*$3.139=$3.14.
I have no idea what idea you're trying to convey here. Are you saying that you've observed an improvement in cruising fuel economy by using high-octane gasoline?
It occurs to me that now that I own a modern car, I could pretty easily plug in ye' olde scanner and do some WOT pulls on both 87 and 93 to see if there's any meaningful difference in ignition timing, VE, etc., between the two.