Octane vs. mpg

Dirtautoguy · June 5, 2013

So I decided to do an experiment with my 1996 chevy k1500 with 3.42 gears using my scan gauge. I usually put the cheapest gas I can find in it which is usually 85octane from a gas station that I have heard multiple reports of ice chunks forming in the gas they got from there. Dumb I know but it gets me around. Today I felt richer and decided to put chevron 87 octane in and in the short 15 miles a drove I seemed to be averaging about 1.5 mpg increase over what I usually get, and got about 22mpg! Going 30-45mph. I drive this pretty often especially lately and I don't think 15 miles can bring any conclusion so ill pay more attention,to the numbers, drive some more, and post what I find.

TooTall1 · June 6, 2013

Best results would be to run ethanol free gas if you want to improve your MPG.

Dirtautoguy · June 13, 2013

Well I have driven the same 15 miles probably 20 times over the last week and I'm still getting around 2mpg better than I was. Sometimes averaging 22mpg! But usually 20mpg going and about 18-19 mpg coming back

Dirtautoguy · June 13, 2013

Plus the engine seems to sound healthier And have a better gas pedal. But that could be my imagination.

AzDragonLord · June 14, 2013

I think the higher octane is helping, but so is not filling with that crappy ice-filled gasoline.

gkhatta · June 14, 2013

When I fill up at sunoco I get 3mpg better then anywhere else. At first I thought it was just in my head but it has been consistent the last 30 fill ups. I drive about 80 miles a day.

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Badass69 · June 16, 2013

Octane really won't do much unless the pcm is actually pulling timing because of knock retard. Now gasoline quality on the other hand will make a change possibly. You need more ethanol laced gas to do the same job as straight gasoline so their would be a mpg difference there. Nowdays a good chunk of premium is just straight gasoline whereas almost all regular brands have up to 10% ethanol in them. That is about the only difference in fuel economy you will probably see on a stock 4.3. Even my 05 with 28 degrees of total advance wide open ( up from the stock 17 degrees) doesn't knock whatsoever on regular grade gas as I have logged it to see. Mileage..... non existent as I don't drive in any fashion to honestly care.

Dirtautoguy · June 18, 2013

My truck has a 350 in it. And I'm still seein the better gas mileage with the less quality gas stations 87 octane.

Badass69 · June 19, 2013

4.3/ 350 who cares, lol. J/K...... sorta.... same engine minus 2 cylinders and for this wouldn't make any difference. Mileage and fuel quality matter a lot more than octane rating. Those "less" quality stations get their gasoline from the same refinery as at least one of the big names do so it's the same base stock just potentially different additives with regards to deposit buildup and so on.

You probably were having more issues from the crap other than gasoline you were getting from the other place than the gas itself. If you aren't losing timing from knock and you have roughly the same quality fuel and ethanol content..... the mileage will be the same.

It's like people thinking running premium in a vehicle tuned for regular will do anything......... again, fuel quality comes into play. But if it didn't knock on regular it still won't on premium and it will try and burn the exact same amount of fuel. The pcm doesn't lean any further than stoich and if it uses the same timing table it won't see a bit of difference.

I am not doubting you saw improvement, but like I said.... it's probably more the fuel you were getting then versus now over the octane level. Vortec 350's have 9.4:1 compression and pretty efficent chambers so with the stock timing table you shouldn't have any trouble with the lowest octance level you can find as long as the gas is clean.

Matte454 · July 9, 2013

Ethanol makes a big difference in fuel mileage, I see the 1-1.5 you see on my 2012 4.8l switching between stations. I spent 3 years in blending and shipping gas so I'm pretty familiar with the stuff. Yes typically all the stations get it from the same place. The unmarked trucks fill from the same rack as the corp trucks, although from different tanks. Month of the year makes a big difference, they adjust the vapor pressure by the month (gov regulated) and summer is better, winter is more butane (cheap/low energy) and worse. The other do side with ethanol is the bob (oxegenated blend stock) that it is to be mixed with is basically 80-82 octane mixed with ethanol to bring it upto 87. The old reg unleaded was a true 87. Some places in the country, islands, places with lots of boats and such still use the old 87 rul blended with ethanol or not. Often you end up with way higher octane then spec at the pump with certain brands that have not retuned their refineries for bob blend stock. Probably what you encountered with the chevron gas, around here the 87 is about 88.5 octane at the pump, and good gas. I have had experiences with Costco gas that was so high vapor pressure (butane) that it ballooned out my Jerry cans to the point they were pretty much round. Also that batch made my truck and car hunt at idle (wife thought it was going to die)

PH-Wolf · July 30, 2013

I always run 93 octane in my truck, yeah I know it's only the 4.3L V6 but I don't care, I like that there's no ethanol in it. I do manage to average better MPG too. Heck I run high octane in everything(some of my equipment requires it as well). The CTS-V 91 octane minimum is required anyways. I noticed my trucks are picky on what brand of fuel too, I get the best fuel econ in both the 4.3L and the Duramax with Mobil fuel. The CTS-V well there's a reason I had to pay a gas guzzler tax when I bought it. LOL. I did get 21mpg on a road trip from Wisconsin to Kentucky last week in the CTS-V.

desrtrat · July 30, 2013

my LQ9 runs best on 91. i have used 87 only a few gallons at a time, and noticed a decrease in performance. in side my fuel door it states use 91+ octane fuel only from GM. so use what is recommended by the manufacturer, or run higher. i would not run lower than recommended for long time.

Chris · July 30, 2013

my LQ9 runs best on 91. i have used 87 only a few gallons at a time, and noticed a decrease in performance. in side my fuel door it states use 91+ octane fuel only from GM. so use what is recommended by the manufacturer, or run higher. i would not run lower than recommended for long time.

Same here I will admit to using 89 a couple weeks ago to pinch pennies

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kstruckcountry · July 30, 2013

I have been running 91 for a while now, no change in mileage but the truck does seem peppier. I'm hoping that a tune will let the truck use the fuel optimally.

When I'm on the farm the closest place for 91 is 30 miles away, so I just fill up when I'm in shopping.

TJay74 · August 19, 2013

Best way to look at octane is run the lowest octance you can that doesnt allow KR/detonation, the lower the octance the more BTU/energy the fuel has. I run 87 in the spring/fall/winter months and then once the heat of summer hits I move up to 89 in my truck, works great for me as I never hear any detonation or pinging.