Higher Octane gas has flame front retarders in it, so it burns
slower. No flash-over or at least less so than regular gas.
        For some reason, although the total BTU's per liter are
similar, published literature suggests that, in theory, a properly
tuned engine should get better mileage from regular if you operate it
within the optimum rpm range. ( near maximum torque levels).
        Much like the old gun powder vs. the new gun powder where the
new stuff burns slower allowing more energy extraction (muzzle
velocity) for the same amt. of powder in a similar length barrel.

YMMV..                  David (TDdsl82westy)

P.S.: Better qualified List member advice is requested here.

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>I've been keeping very good records of my gas usage over the years
>.. lately I'm been experimenting with 91 vs. 87 octane gas ... so ...
>
>I traveled 743 kms on 86.556 liters of 91 oct ... and ... 770 kms on
>92.827 liters of 87 oct ... this is an average over two fill-ups
>(I've averaged it over 3 and 5 and get pretty much the same numbers)
>.. I get 8.584 km/li, or 20.52 mpg U.S. or 24.24 mpg Imp on 91 oct
>and 8.245 km/li or 19.71 mpg U.S. or 23.29 mpg Imp on the 87 oct ...
>the problem as I see it is I did not control for speed ... I know
>that drive faster with 91 oct than with 87 .. and I generally drive
>fast in the 78 (120 to 130 km/h) and maybe 80% of my driving his hwy.
>
>So ... I travel 0.339 km further per liter, or 0.81 mi further per
>U.S. gallon or 0.95 mi further per imp gallon ... on 91 oct vs. 87
>oct .