91 octane vs e85 in flex fuel Tahoe/suburban?

Which of these is better? I wanna save money of course but if 91 octane overall will increase longevity of the vehicle than by all means

Idk if it matters but the winters are harsh and the summers are hot here.

If you are doing it for cost, it depends on how many cents per mile it costs to run each fuel.

I would not run E85 during the winter, it ought to be E70 for winter but can still be hard to start.

The motors love E85 and take 5 tanks or so to get fully acclimated to it. The engine makes more power, idles smoother, cooler and even the transmission runs better. It's literally like cheap race gas. Oh, it's also a great injector cleaner.

When we are not planning to travel, we run E85. If we need the range, we run 93. I never intentionally run low octane fuel.

Here are the stats for my Yukon XL with the 5.3 motor:



You can see that for all city driving and 70% highway driving for a tank of fuel, the mpg difference is about 20%. Eventually, the faster you go the greater the spread between the two fuels cost wise.

There's no need to run 91 octane, unless you have the 6.2. Any benefit running a higher-octane fuel than what's required in your owners manual is a persistent internet myth that won't die. 5.3? Run 87 all day every day.

If you have a yellow gas cap, you can run E85, and you might notice a little more power from the engine. You'll also see a significant decline in mileage, likely greater than any cost save.

E85, or more specifically the alcohol in E85, is hygroscopic - meaning it absorbs water. So I would expect some long-term impact in terms of possible/likely increased corrosion in the engine due to the extra moisture.

iboughtatahoe23 said:

Which of these is better?

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What do YOU mean by 'better'? I know many 'kids' who are perfectly willing to pay MORE for E85 than for 87,
because E85 is moar powa, yo - plus it burns cleaner than petrol.

iboughtatahoe23 said:

I wanna save money of course, but if 91 octane overall will increase longevity of the vehicle, then by all means

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One of my close personal friends (a professional pcm / ecm / tcm tuner)
stopped paying for 91 octane once it cost 'too much' compared to 87.

My rule of thumb is that if E85 costs AT LEAST 17% less than 87 octane,
then it MAY be worth it to buy E85 instead of 87.
I.E.: In NYC E85 is nearly the same cost as 87, so I usually use 87.
In Long Island, E85 is cheaper by enough that I'll use E85.

iboughtatahoe23 said:

Idk if it matters, but the winters are harsh, and the summers are hot here.

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Unfortunately, E85 in the winter is E51 - it's only E85 (E83, actually) in the summer.
I wonder what it is in the spring and autumn?
In NYC, price relationship to 87 is pretty much the same year round, which is lame.

On the 5.3L e85 is great as scrubbing bubbles for the t/b and fuel system to remove carbon deposits. When it’s available and priced right, use it. In SD maybe not in the Antarctic months.

On the 09-14 6.2L that need premium, e85 is a magic bullet. Here in SoCal it’s the difference between a $75 vs $140 fill up. Milage drops 20% in city traffic. The motor does downshift faster and runs smoother.

When my daughter got my grampas 01 flex fuel S10 with 32k miles, the 2.2L 4 cyl motor crawled up hills and knocked. We ran e85 for the first time in 23 years last year.

It woke up all 140hp in that tiny Iron Duke motor and stopped its detonation. It finally downshifted going up hills and Rev’d freely through all 4 gears of its 4L60e. Day & night difference. Lost 40 miles per tank in range city driving but saved $20 over reg fuel price per fill up.

Lots of good information already provided, so I'll just ad a tidbit more:
1. As stated, unless your 5.3L can run E85 from the factory, you won't see much of an improvement because it's not a high compression motor like the 6.2L. But, you may feel a difference just because of the way E85 burns in the engine.
2. I've been running it in my 2008 6.2L now at a blend of appropriately 30% of total fuel used (if the computer says inused 21 gallons, then 7 of E85 get put in first). Then top the tank with 87. This has proven to be a fun, economical (both cost per mile, and smiles per gallon) way to wake up this animal.
3. The cleaning characteristics are fantastic, and as @swathdiver stated, it takes a few tanks for the ECM to acclimate to it.

So, overall, it won't hurt your vehicle. If you don't have the flex fuel designated engine/PCM then I would experiment with some blend ratios and see what feels good to you and math's out to your liking. I've found a sweet spot for the driving I do with my blend - and some slight tuning of the spark tables and trans shift points.

Now that’s it’s in the cold 40’s here in SoCal, rich start ups on e85 smell like a dirty martini parts cleaner rather than a half cup of 1960’s non catalytic exhaust.

swathdiver said:

If you are doing it for cost, it depends on how many cents per mile it costs to run each fuel.

I would not run E85 during the winter, it ought to be E70 for winter but can still be hard to start.

The motors love E85 and take 5 tanks or so to get fully acclimated to it. The engine makes more power, idles smoother, cooler and even the transmission runs better. It's literally like cheap race gas. Oh, it's also a great injector cleaner.

When we are not planning to travel, we run E85. If we need the range, we run 93. I never intentionally run low octane fuel.

Here are the stats for my Yukon XL with the 5.3 motor:

View attachment 444786

You can see that for all city driving and 70% highway driving for a tank of fuel, the mpg difference is about 20%. Eventually, the faster you go the greater the spread between the two fuels cost wise.

Click to expand...

Thanks for the info. What’s your opinion on E-30, more specifically in the winter. And is it bad to switch between fuel types each tank just seeing if there’s any differences? I’ve tried e85 (my go to as it’s cheap) I’ve tried 91 octane, e87 and e30. I feel like 91 idles smoother but could just be placebo.

E87? Do you mean 87 octane?

Gasoline, unless otherwise specified, tends to be G90 / E10: 90% petrol, 10% ethanol.
Some gas stations are even trying G85 / E15: 85% petrol, 15% ethanol.
Very few gas stations serve G100, ethanol free gasoline, luckily you know when you're getting it, they brag about it.

Never seen E30, or E87.