Unleaded Plus vs. Unleaded Premium

[Hops]

Originally Posted by Rob-00BoxsterS

Last year I did a test where I filled up 10 tanks with 87 and 10 tanks with 93. I started the test with the tanks completely filled and ended them filled. I then averaged all 10 tanks so the differences in volume should be very minimal.

Did you run 10 tanks of one octane followed by 10 of the other, or did you switch back and forth in some manner?

The extreme of 10 of one then 10 of the other is the least valid because the variation in temperature, season, mechanical factors (tire pressure among others), and gasoline blend would be difficult to control.

At the other end of the spectrum, alternating 20 tanks isn't a great idea because you are blending the two octanes to some extent with each fill (so the actual octane is somewhere in between). Also, I don't know for a fact how long it takes the car to completely adjust to the new octane... is it instantaneous (my assumption), or does it take 5 miles or 50 miles? (Insight on that question is welcome if anyone knows.)

If anyone plans to run this type of test, I might suggest running three tanks one and then three tanks of the other and repeat this for three or four cycles. Make some note of your balance of highway versus stop-and-go driving for each tank, check tire pressure with each fill, and note general patterns of high temperatures (for instance, 50s / 70s / 90s). We could draw some conclusions out of that sort of data.

I won't personally be doing this... I'm in CA with the highest octane of only 91 (aka craptane). I'd be happy to test 91 versus 93 or 94 if that were possible but have no interest in testing 89, which saves only ~12 cents a gallon and is below Porsche's minimum recommendation.

[Rob-00BoxsterS]

Originally Posted by Hops

Did you run 10 tanks of one octane followed by 10 of the other, or did you switch back and forth in some manner?

The extreme of 10 of one then 10 of the other is the least valid because the variation in temperature, season, mechanical factors (tire pressure among others), and gasoline blend would be difficult to control.

At the other end of the spectrum, alternating 20 tanks isn't a great idea because you are blending the two octanes to some extent with each fill (so the actual octane is somewhere in between). Also, I don't know for a fact how long it takes the car to completely adjust to the new octane... is it instantaneous (my assumption), or does it take 5 miles or 50 miles? (Insight on that question is welcome if anyone knows.)...

I didn't switch back and forth but I did run two tanks of regular (and discarded the results) before starting the test on the regular. The super was what I normally used so no data was dropped.

I realized I was not taking into account many factors but I did try to be as consistent as I could. Tire pressure was checked every weekend (habit I have, not just for the tests). There was a few times I need to drive a substantial distance on the interstate so I took a different car so as not to throw off my results. All traveling was done in town (but even that could be considered inconsistent) and the 20 tanks took me a good 5 to 6 months to complete so temperature would have been a big factor it didn't account for (although it is always hot in Florida .)

Originally Posted by blue2000s

I know you mentioned that your test wasn't scientific. I appreciate your admission. Fuels have different formulations at different times of the year, and economy is heavily effected by the air temperature and humidity as well as the other factors you noted.

That is one of the things I really like about this forum. People here are usually so respectful when they post a reply (I mean that sincerely, I'm not being sarcastic. ) You are right and I didn't even think about different formulations or humidity.

My apologies for not being more thorough but when I did this, I was not expecting to pass my finding off as fact. I did it more out of curiosity because my wife was getting such bad fuel economy in her Land Rover and she was too thrifty to put in 93 octane.

[Hops]

Originally Posted by Rob-00BoxsterS

People here are usually so respectful when they post a reply
...

My apologies for not being more thorough but when I did this, I was not expecting to pass my finding off as fact.

I agree with you. I've only seen a few people bicker on here, and I don't understand the point of taking time out of your day to go online and bicker with someone. This is by and large a very friendly forum.

Thanks for providing a little more detail on your method. I am glad you ran your tests. None of the rest of us have gotten off our arses and done it. If I come across as an analyst or something, well, I am. It would be cool to get enough data with enough variables trapped to really get a read on how big of a factor octane rating is in the overall fuel economy of our cars.

Unfortunately the number one factor is our right feet. I've done as high as 28mpg and as low as 16mpg in my '05 987S. As I think about this a little more, the best measure may be mpg over a controlled course using the onboard computer. So, driving multiple runs on a defined section of highway with the cruise set at 65 (or whatever) and resetting the onboard mpg at the beginning of each run might be the best apples to apples comparison... and it wouldn't take months to complete. Of course, this doesn't necessarily reflect the mpg difference in normal driving patterns, but it would be a start.