What if someone ran 85 octane (at low elevations, not high elevations where it's considered "regular" gas)? How about 83 (assuming it could be purchased)? Would that lower gas mileage and performance? If not, why aren't we being offered that? If so, then why wouldn't raising octane, to a certain point of no return, raise gas mileage and performance?

The problem isn't that octane doesn't affect gas mileage and performance--it certainly does. The problem is figuring out at what octane this stops happening. The Jeep manual says 87 and I accept that. Try putting in too low an octane, and if Jeep is like most recent engines, the knocking will be sensed, the timing will retard, and gas mileage and performance will drop. That's why you can't just buy gas that doesn't knock. The computer tries its darndest to keep that from happening, but at the expense of mileage and performance.

Now, who can prove they ran 87 octane in their tank? How? Are you going by that state standards board sticker on the pump? That's the first place I'd look if I confirmed better performance from an octane rated higher than the manual says: The gas station. What are they doing to their gas? If you're interested in how to check that gas for illegal dilution, start here:

Auto Clinic Expert Q&A

And, BTW, while in Moab (higher elevation where 85 was considered "regular") a couple weeks ago I had to buy premium gas in order to get performance--the loss of horsepower/increased pinging was that noticeable over three tanks of gas.

--mark d.

 

That is a good point made about actual octane rating. We as consumers can only hope that the companines we are buying from are honest enough to sell us what they say they do, and we can only hope those who are regulating are doing a good job of it!!

I do know that there is a chain here in WV called Speedway, i know they are elsewhere too, but they always have gas 3-5cents cheaper than everyone else, and the DO have a higher % of ethanol than most other stations. Their district mngr told me so!! I always try to buy name brand fuel, usually BP or Exxon.

All I know, is with MY Jeep, it is MY observation, that it runs better and smoother with 89 Octane. Whether the fuel mileage is actually better is still waiting to be determined!! I'll find out here in a few days when I fill up!!

 

If you read the pumps, they state a minimum octane of 87. With that said it has been reported many times that you can be getting 89 or 91 etc.. You don't know for sure what you are getting. Was it diluted by the residue left in the transport from the last time it was filled? Too many variables to the consumer to just say it won't improve or it will. If an engine gets carboned up, it will run better and possibly get better gas mileage due to running higher octane (slower burning) because it will slow down pre-detonation caused by heatsoak in the carbon buildup (trying to deisel). If all things were new (no carbon build up) and you tested and were sure of actual octane, high octane should not perform better or get better mileage than 87 if the vehicle was tuned for 87 (knock sensor range should handle most conditions).
My $.02 + $3.00 might get you a gallon of gas.....87...89...91 depending on the day/market.
Jim

 

i've gotten the same results by using different gas stations. i was getting mid-15's using the same station regularly and switched to a different station for 2 consecutive fill ups and got slightly better mileage with the same octane. there are also way too many factors that go into this slight result than just the gasoline and it's octane rating that unless you have a controlled environment it's hard to even want to starting thinking about (though obviously i have, huh). what was the weather? wind? humidity? hills or flat? same route taken with each octane? stopping at the same lights? accelerating at the same rate with each tank? tire pressure constant? passengers or cargo? for a just one or two mpg the list of possibilities are endless.

 

Here in Iowa, it is just the reverse for 87 and 89 Octane. 87 Octane generally has no ethanol added, and the 89 Octane (midgrade) does.

The 89 Octane is also usually 10 cents a gallon less than the 87 Octane (have seen it for as much as 15 cents a gallon less some places).

I run fine as far as performance on either, but I get better MPG from the 87 Octane...usually 1-1.5 MPG more....not significant, but a little better.

So if you figure 1 MPG less, for a 18 gallon fill, I went 18 miles less, but spent $1.80 less to fill it. With the price per gallon at well over $2 a gallon (closer to $3, but anything over $2 works for this equation), with an average MPG of 18-19, the 87 Octane is the better choice for fillup.

 

OK... I give in to curiosity...

After repeatedly asserting that higher-octane fuel is a waste of money in the JK, I decided I may as well try it firsthand before condemning it... so I just filled up with 89 octane and will track my mileage faithfully (as I always do - 8500 miles worth of gas logged so far in Excel). I'll report back at some point (most likely in a different thread).

 

Rather than the octane effecting the mileage, it could very well be the brand/quality of the fuel. I've had two highly tuned Mustangs that were extremely finicky about the brand of fuel they ran. Based on this, I've determined that some fuel suppliers don't necessarily sell the quality of fuel I'd like to be running in MY vehicle. Mileage and performance both suffered when "bargain" fuel brands were run (not ethanol added...I'd have blown my engines if I'd ran that crap).

Is the engine in the JK as picky about brands as a highly tuned Mustang engine? Certainly not, but it could be playing a part in the difference in the "butt dyno" and the mileage calculations.

So, if you want consistent results, get the best quality fuel available in your area to base your comparisons on.

 

Ok, here's what I came up with. This is just my Jeep, I'm not suggesting anyone else's Jeep will do the same, but, here is the conclusion I have come to.
I filled up today at 1/2 empty. I normally run till I have 1/8th tank and then fill up, but I saw where this was heading. I was at 182.5 mi, since my last fill up and right at a 1/2 tank (my last tank of 89, I was around 215mi at a 1/2 tank ). On 87 octane 182.5 mi./11.3 gal to fill=16.1 mpg.
My last fill on 89, 313mi/18.1gal to fill=17.3 mpg. Better fuel mileage along with a smoother running Jeep, no stumbling like with 87!!
Same brand of gas bought at two different stations, but both were 711's that have Exxon filled by the same petrol supplier here.

Obviously, the "exact" same routes were not taken with both, but driving conditions were similar. To and from work, I work in marketing, so I visit clients all day, back to work, then home. No AC was used for either tank. Both were approx. 80% city, 20% highway at 70-75mph.

Do what you want with your Jeep, do what you want with this info. Im running 89 in mine!!

 

4th here. My Mom gets around 21-23 Combined with 89.

 

22.5 with 87