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 .