In another forum, somebody just posted that they got way better mileage with a higher octane gas (in their case, from 86 to 88). They claimed that the overall savings, based on increase in MPG, was like $0.20 per gallon.

Of course I'm sceptical. So I was wondering if anybody here has had any experience with trying slightly higher octanes, and what the difference might have been, if any. Here in CA, I have heard that the 'winter blend' of gasoline gives poorer MPG, and so if the higher octane story is true, then it seems that this might make up for the mileage differential.

 

I should point out the the car in question was a HCH, and the theory was that the car's high compression engine's anti-knock sensor was kicking in and hampering the efficiency and thus the MPG.

The writer mentioned their MPG going from 47 to 54.

 

Hi Chuck:

___First off 54 mog after a few tanks in a brand new HCH doesn’t mean much considering what some of the pro’s here get tank after tank after tank on Regular unleaded. Secondly, you already know my view point as posted in “the other forum”

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___Waynegerdes@earthlink.net

 

This one has me skeptical and intrigued both.

I am filling up tomorrow with Super unleaded (91) and I will report my MPG after that tank. I have been getting a pretty consistent 44-49, so if this tank is better, I will have a good basis for discussion.

UPDATE:

After 40 miles on this new tank with 91 Octane, I am at about 62 MPG. Higher than I have ever achieved at that many miles into a tank. So it is getting interesting !!!

 

Found this in a PDF file today. Pretty amazing if true, and I have no reason to doubt them.

Total Energy in a gallon of gas.

13% runs the car, consisting of 3% aerodynamic drag, 4% rolling resistance loss, and 6% braking resistance loss.

87% is wasted, consisting of 62% engine loss, 17% idling loss, 6% drivetrain loss, and 2% accessory loss.

See it in this PDF, on page 7:

http://www.ef.org/documents/2003AnnualReport.pdf

 

These figures might be correct at a certain (slow) speed but I'm almost certain that aerodynamic drag consumes the biggest percentage of the power avaliable from the engine.

 

Wow, that is indeed interesting. After a full tank, if you get anything close to those results, it would be even more remarkable.

I attempted to run my own test over the weekend, but my test was disrupted by: (1) huge wind and rain storm, and (2) spouse driving car for the first time. I will have to attempt another test under 'normal' conditions.

 

In the "old" days, higher octane meant higher density gasoline due to higher reformate content, but in the new world, higher octane is sometimes achieved by addition of oxygenates which are not only less dense, but have less energy per unit of mass. On balance, unless the octane is so low that the knock sensor is retarding the engine timing, I am not sure that the answer to this question will ever be clear. One thing that is certain, if you use gasoline with ethanol added, as is common in the midwest, you are very likely to get lower mileage with higher octane gasoline.

 

Update again: After 92 miles on this tank, I am at 58.6 mpg. I lost about 1 MPG this morning on the cold engine commute. Temps outside were about 48 F. I have never had numbers this high after 92 miles on a tank, so I will keep the board posted on the ongoing results.

 

OK I did some math to try and see "where" buying higher octane gas would pay off financially, if indeed it pays off in higher MPG.

It boils down to this: with the difference in the octanes being 16 cents per gallon, it would be cheaper per mile to buy the higher octane ONLY IF the higher octane consistently provided AT LEAST 5 more miles per gallon per tank than the lower octane.

I'll keep everyone posted on the results of the higher octane tank.