Here3 in the U.S. they post an average of RON + MON figured exactly as (RON + MON)/2
More than half our gasoline sold is E10 which means up to 10% ethanol and, unless your car is mid 2000s or newer, it will usually disagree with a few things that the newer computers can adjust for.
In scientific testing terms ethanol has a higher octane rating than gasoline
In scientific testing terms ethanol has a lower energy content than gasoline
They can legally take a lower than 87 octane gasoline and mix it with 10% ethanol to raise the advertised octane number. A lot of the mid-grade and premium gasoline sold is little more than regular with the ethanol factor added in.
Even 100% gasoline varies widely by the refinery that made it and what base oil stock was used. We have a few stations that have absolutely outstanding 100% gasoline and a few whose gasoline sucks even at 100% pure.
Crude oil is classified as sweet or sour, you'll have to look up the difference.
Gasoline made from sour crude is usually sub-par even on an equal octane and no ethanol comparison.
Gasoline made from the best crude oil stock tends to be the best, their 87 octane may not knock where others will.
Sub par gasoline usually smells odd. It smells old. it smells like it has some diesel fuel in it or just smells funny.
(yes, adding diesel fuel to gasoline somewhat increases the rated octane properties but only a few percent can be added)
Good gasoline smells clean, crisp and volatile. You'll notice strong hints towards a decent concentrate of toluene &/or xylene in it.
The more it leans towards the xylene smell the better it seems to be but the toluene stuff isn't far behind.
Truth of the matter is I've got one station that consistently provides excellent gasoline and I try to fill up there as often as possible. It generally costs a little more but the returns in mpg and driveability are well worth it.
If I'm stuck close to empty and have to buy gas I've got a few alternates I'll settle for and one of those is even an E10 gasoline, it must start out with good base stock before they throw the vodka in it. Oddly enough, at any station including the ones I know are historically acceptable, I'll pump a small amount then intentionally get some on my finger to smell it. If it doesn't smell clean, crisp and volatile I'll just pump enough to get down the road.
I didn't used to be so critical until I built up that 98 GTP Pace Car and had the tuning optimized for 95 octane but would still accept a good 91. This was actually a car that, if you goofed and ut 87 in it, you babied the thing over to one of the few stations we've got that sells racing gasoline in 107 unleaded and (if you bring a can) 114 leaded.
It's also true that running a higher octane gas than your engine needs gains you nothing but perhaps a bit more detergent additives.
Our bikes were built during the days when "regular" pump gas was usually 87 - 89 octane - at least over here.
