You're right, I made a mistake and wrote septane instead of heptane.
"The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane
The best fuel economy you can achieve in any vehicle is by running the lowest octane fuel before timing is pulled (retarded). This isn't my opinion, it is fact.
The reason more power CAN be made from higher octane gas is because the engines attaining the power are more efficient, not because the gas is better. Higher compression engines are more efficient than lower compression engines, but they are limited by knock/ping. It's common knowledge that higher octane resists pinging, that's not news to anyone.
NOTE: the following is for illustration only, not accurate (unless I got lucky with the numbers). Lets say a litre of 91 has 30k btu's, and a litre of 87 has 32k btu's. An engine that runs 91 efficiently may usefully turn 40% of the 30k into power. An inefficient engine may only turn 30% of btu's into power, but it starts with more. This is where the problem comes from as far as power is concerned. Very few people know when their timing is being pulled, and never have I seen a dyno sheet with timing and air/fuel comparing gas grades in a test vehicle.
As much as I hate anecdotal evidence, I know from my regular commutes that I get the best mileage with 89 in the summer, and 87 in the winter. I also track my fuel consumption very religiously on gas cubby (iphone app) and have tried every gas brand, and grade I could. The best I ever got was 7.4L/100kms (Pioneer, 89, summer) and the worst I ever got was 10.2 (Pioneer, 87, winter - this tank was all very short trips in city). The worst summer mileage I ever got was Mobil 1, in Michigan, 89. It was so bad I thought they had water tanks in ground that they mixed with the gas. My average is 8.5L/100kms over the course of the last 50,000kms (~17months). Also, I speed. 100kms/hr is a lullaby not a speed limit on a 400 series highway.