After I started working at my current company I soon learned a dirty little secret of the petroleum industry: don't buy mid-grade gasoline; it's a rip-off.
Mid-grade gasoline (plus gasoline, or whatever it's called in any given location) is usually priced right between regular gasoline and premium (super) gasoline. If regular is $2.99 per gallon, mid-grade is usually $3.09 and premium is $3.19. (I can't remember the split for regular, mid-grade and premium gas in Canadian cents per litre in Canada.)
Mid-grade prices are usually exactly half way between regular and premium, meaning you would get the same price if you filled half your tank with regular gasoline and half your tank with premium gasoline.
Many gas stations create mid-grade gasoline by pumping out of the regular tank and the premium tank, simultaneously. A lot of American stations got rid of their mid-grade tanks in favour of regular tanks back in the 90s when the EPA was cleaning up gas stations.
There are stations that still have a mid-grade tank. Fuel terminals still sell mid-grade gasoline. Not all oil jobbers buy mid-grade gasoline. Some simply buy regular gasoline and premium gasoline and pump it into the same tanker trailer compartment. This is called "splash blending".
As you can see, there's nothing magic about mid-grade gasoline. It's just a mix of regular and premium. That's not the dirty little secret. The secret has to do with the ratio between regular and premium and how it affects the price.
As I mentioned above, the price of mid-grade is usually right in the middle of the price of regular and premium. 100 gallons of mid-grade is the same price as 50 gallons of regular and 50 gallons of premium. That's not the ratio used by most stations. Most stations use 60 gallons of regular to 40 gallons of premium, or even 65 gallons of regular to 35 gallons of premium, depending on the ratio allowed by the state (or, I'm guessing, province). Michigan, for instance, allows 65:35.
In order to keep prices below some artificial — but psychologically significant — price point, stations have sometimes kept to the 10 cent per gallon difference between regular and mid-grade but lowered the difference between mid-grade and premium, meaning that mid-grade is an even worse buy. I have seen gas regular sell for $2.85, mid-grade at $2.95 and premium at $2.99 per gallon in an effort to stop premium slip over the $3 per gallon mark.
If you buy mid-grade you can save yourself some money by purchasing regular and premium gasoline separately and mixing it yourself. You wouldn't be doing anything that the gas stations weren't doing themselves, except at a better price.
[Edit: I was rushed when I made this post. I changed it a little to make it more international (i.e. I made a reference to Canada) and I changed the wording of some of the sentences. Oh, and I changed "by" to "buy". Stupid reliance on spell checkers...]
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