While this is pretty much un-scientific I still found it interesting.

First the truck - 2017 Lariat 3.5EB, 3.55's and a 36 gallon tank.

After the first 2 tankfulls of Regular (87 octane) it was clear that it just wasn't running smoothly so for the last 28K miles I've been feeding it "Plus" (89 octane) and it seems to run OK. Fairly smooth but a little ratty in really hot weather when accelerating hard.

Around town mileage in warm weather is in the neighborhood of 18 mpg and on the highway at 70 mph with the cruise and A/C on I can squeak out 23 mpg.

Recently, I was down below a quarter tank and being in an inquisitive mood I decided to fill up with Sunoco Ultimate (93 octane) just to see what effect, if any, the additional octane would have. After a couple of hundred miles on the highway at 70 mph with the cruise and A/C on my mileage was hovering around 20.1 mpg!
My around town mileage dropped a bit to 17.5 but it accelerates noticeably smoother in hot weather.

Considering that it was a $110 fill-up and my highway mileage actually went down I think I'll stick to filling up with 89 octane "Plus".

Live and learn.

 

Yep, you’re right......totally unscientific.

 

I'd have to agree. I'd love to see the same test with more science on it.

 

I'm not really sure what science I could add short of a chassis dyno but I do know that I paid $0.25 per gallon additional for fuel that gave me worse mileage. That was all the science I needed.

 

A single fill up is inconsequential, no matter the results.
Temperature, humidity, wind, route, traffic, and driver inputs variance negate any conclusions you may have reached.

Having said that, if you feel better about using 89, then continue using 89. All those octane ratings are acceptable for your engine.

 

A light headwind could have caused it... What I find interesting is how quickly my MPGs drop under any changes.

An interesting result, but completely worthless from a reality perspective. I vote for running any fuel that makes your truck run better, but if our trucks got better MPGs in the real world we’d all be using the same octane.

 

In a factory configuration, there's no need to run anything beyond what Ford recommends. You will lose efficiency with higher octane fuel because it contains less energy/lbm. That's science.

 

So true, running E85 which is considerably cheaper than Shell 93 Nitro by almost a buck a gallon I'm losing about 3MPG, but then again the Coyote runs like a beast on E85 and I'm not easy on the skinny pedal.

 

I’ve never heard there is less energy in higher octanes. My impression was octane and energy in the fuel are not linked. Some Googling, past all the forums, suggest some fuels may have less energy in higher octanes. Huh...interesting. But the increased compression allowed (when engines can manage it) my offset the energy enough to produce more power. So, it’s a very true example of “individual results may vary”.

A link to the article I found: https://www.roadandtrack.com/new-car...-fuel-futures/

Personally, I only run high octane. Never seen a MPG difference, but my engine behaves better at lower RPMs when I’m fighting the Ford shift logic.

 

Shh!!! Don't tell anybody, they'll start charging MORE for 87 mule-**** laced with corn-squeezins.

 

My truck runs best on Chevron 93.

 

The bolded statements are objectively false. It's not that higher octane may have less energy/lbm; it absolutely does. Individual results with like equipment does not vary when comparing fuel consumption rate change with the same fuel at different octane levels. Increasing octane in a gallon of gas reduces the amount of btu's in that gallon. That's what makes e85 worthless for most. While it costs less than e10, it is not enough to offset the additional fuel that must be dumped in to do the same amount of work.

The advantage to e85 is the increased resistance to predetonation, allowing for more extreme tuning than would be possible with e10. When going that route, higher volume injectors and possibly pump must be used due to the additional fuel that must be dumped into the cylinders.

 

You'll get some more power as the PCM adapts timing based on feedback from the knock sensor, but I've never noticed a difference in fuel economy.

A few years ago I had a 2011 Mustang with the 5.0 L, which carried a higher horsepower rating for premium fuel. I experimented back and forth on a few road trips as well as during my daily drive and never noticed a difference. To be fair, I never noticed much of a power difference either… But that thing was a missile even on regular gas. I used to run my work cars on E85 when available because of company preference, and I could definitely feel difference in the Focus.

 

You may pick up power but only if the vehicle is capable of advancing timing beyond what the fuel allows for. If that is on 87, switching to 89 or 93 will not make a difference except the decrease in fuel economy. You not noticing a change is understandable but irrelevant. Everything else there is conjecture.

 

I probably should've posted links, but it's a heck of a lot more than conjecture. There are dyno charts all over the Internet of modern vehicles gaining power with higher octane, and that goes back more than just a few years.

Not only does afford Ford rate power differently in the 5.0 L Mustang, it bears out on the dyno: https://themustangsource.com/forums/f726/87-vs-93 (Web view)

But not all vehicles carry different rating. Mike Harrison, the chief engineer for the relatively stone-age 6.2 L V-8 mention this nearly 10 years ago during a Q&A thread in the super duty forum: https://www.ford-trucks.com/forums/9...ml#post8833938