I acknowledged already that I posted in the wrong forum. As for the test conditions, I also mentioned in my post that the temperatures were all over the place, and my driving speed and traffic conditions varied over the trip. Without some sophisticated logging equipment or a passenger with a clipboard, I don't think I could have done much better.

Still, I think it's interesting that the fuel economy numbers only vary by about 10% over the course of 3000 miles and eight tanks, and the economy measurements seem to be completely uncorrelated to octane rating. In fact, both the "best" tank and the "worst" tank were when I was using 87 octane.

And although I only posted one set of numbers, this isn't the only time I've done this. There was another trip two months prior to the one I posted that was from Virginia to Texas and back, with similar results. And in December, I drove from California to Texas and back, again with similar results.

I feel pretty confident that for *my* car, at least on highway trips, any difference in fuel economy resulting from octane choice is totally swamped by the normal variations due to temperature, altitude, driving speed, etcetera. I still use 91 for my daily-driving (91 is the best I can get here without driving 50 miles to get 100 octane), because it's only $0.20 more a gallon than the 86, the owner's manual recommends it, and I realize that my highway observations aren't really relevant to my daily driving conditions.

 

You know, in a for-what-it's-worth thought, you were always running a somewhat richer mixture than 87. After all, when you ran an 87 tank down, you added 91 for a mix. And when you ran a 91 tank down, your fresh load of 87 would have been diluted. Might not have been a big difference, but just a couple of octane is the difference between the grades.

And if you put too low an octane in the tank, the best thing you can do is run it down a bit and dilute it with premium. We had to do this when we had a BMW 2002, back in the day, and got some gas that was clearly lower than the tank label (pinged at any speed, including steady cruising). After 50 miles we added some premium; problem solved.

Then again, it was only 26 cents a gallon... Typical price, back then. Oh, for the good 'ol days!

 

I don't use 91 octane to increase my mileage. Not sure the difference in octane rating between 87 and 91 would really matter considering all the other variables associated with it. It's clear the people who designed the engine suggest using 91 octane. They certainly know more about it than me. It's between $2 and $3 more per fill up (about a week for me). If using premium, and those couple bucks more was an issue for me, I would have bought some other car.

 

I've had my MC for 2 weeks now and just filled up for the first time.....used regular. I'm about to go on a road trip and will pay attention for any reduction in performance, but am confident that it will run fine on "87 AKI" which is the same as "91 RON" (2 different methods of rating octane).

 

Check your owner's manual. If I recall correctly, mine specifies to use at least 89 AKI, and recommends 91 AKI or better.

 

https://www.northamericanmotoring.co...630-post9.html

 

89 minimum and 91 recommended.

 

Does anyone know who I could email at MINIUSA to find out the actual minimum octane requirement for a 2013 "justa?"
Apparently 89 was not the minimum requirement in 2010:

https://www.northamericanmotoring.co...05-post59.html