[JimD]

I keep log book in my car and write down each fill-up. My bimmer is a e88, n52. For the first 58K+ miles, I used at least mid-grade gas. I used premium if running an autocross or something but used mid-grade for regular use. My mileage averaged 26.69 mpg.

Recently I started using my bimmer as my daily driver. I switched to regular since BMW says it is the minimum acceptable octane. My plan was to switch back if it pinged at all or showed any other sign of an issue. I have detected no issues so I have now used it for over 13K miles. My mileage over this time has averaged 27.4 mpg. If I run an autocross again or otherwise anticipate needing maximum power, I will fill up with premium. But for my daily commuting, I think regular is working fine.

I also changed plugs recently - at over 70K miles. The gap was surprisingly large on the ones I removed. I should have measured it but did not. On the next 4 tanks (about 1200 miles) I averaged 25.79. On the next three tanks I measured 27.4 mpg - exactly my overall average since I switched to regular. Do you think it's possible our cars learn to change the spark advance or something to adjust for the gap in our spark plugs?

I get frustrated at times with my bimmer but I should also give the BMW engineers credit for the spark control technology. I got rid of my Suzuki SUV to get down to one car. It was supposed to be able to burn regular but it pinged in really hot weather and when I was pulling a heavy trailer. So I had to use higher octane fuel occasionally. I detected a small mpg benefit in that vehicle but not enough to justify the price of the higher octane fuel. I don't haul anything really heavy with my bimmer but it has a hitch and I pull a trailer occasionally. I have never detected pinging. I believe BMW has better logic controlling the engine to make it work this way.

BMW recently ran a silly article in their weekly email saying higher octane is better for our cars and would justify itself with the mpg savings. In my car, that just isn't true. He cites better cleaning but that is a function of the brand, not the octane. I think it is good to test higher octane, especially if you detect pinging, but if the performance doesn't change, I believe it is a waste of money. The higher heating value, the measure of the energy contained in the fuel, is actually higher with lower octane. So if your engine burns it OK, there is a reason mileage could be very slightly better on lower octane. But not if the engine computer has to back off on spark advance.