In Reply to: Does octane affect mileage inversely? posted by Marketing hype? on December 10, 1999 at 16:49:19:
Here's my best engineering explanation, but the short answer is yes, it is a myth that high octane fuels give better economy:
The octane rating of a gasoline is a measure of its resistance to knocking (ignition of the fuel prior to the spark). This causes the gas to explode rather than burn, which can be audibly annoying and results in greater stress on the bearings and other engine parts which over time can be harmful. However, unless the problem is extremely severe it won't have any noticable effect on economy.
The way manufacturers increase the octane rating is by adding a chemical, I believe it's called toluene, to the mix. This chemical does not store as much energy by volume as gasoline, so adding any of it to your gas will reduce is efficiency. I'm surprised you would notice a difference, since the additive ratios are pretty small, but the truth is the higher the octane rating, the less thermodynamically efficient the fuel is.
What I don't understand is that BMW's are supposed to have anti-knock sensors, so it really shouldn't matter what gas you use. The only effect might be that if you use low octane fuel the sensor will retard the spark, resulting in slightly less power.