The 4runner runs fine on 87, however I have done math via Fuelly, it gets roughly 10% more efficient with 91.

So it’s a simple math exercise, if premium is less than or equal to 10% more than 87, it gets premium.

We usually only fill at costco, so she almost always gets premium.

Okay in fairness this is a confusing topic. In Japan where these cars are made, they use RON whereas in the US we use AKI(RON + MON / 2).

So the manual says use 87 octane or 91 RON.

87+91/2 = 89, otherwise known as mid-grade.

This^^

87 is just fine. High test is for turbos, superchargers, etc... stuff with high compression. Your "increase in mileage" is due to the lower weight of your wallet on 93.

That seems like creative math to me. The manual says 87 and the pump says 87 and my truck has run fine for 15+ years and 232K miles. I’m not sure there’s anything more to it.

This made me look closely when I got gas today. 87 - $3.39, 89 - $4.35, 93 - $4.59. 94cents more to go up 2 points, I'll just stick to 87.

In my case, it only cost 30 cents more for 91. Maybe Ill try premium in my next tank and see how it goes.

We get gas at Sams Club because its usually a bit cheaper than costco, but I just checked the prices at both and they're the exact same. If its the same when i needa fill up again, I may go to costco instead considering they have "top tier" gas unlike sams club.

Absolutely, in Alberta land, if Costco 87 is at 1.50 per litre, 91 is only ~.08 more, so well worth it by my experimentation.

Challenge: find one independent website that states higher octane fuel improves mileage. It doesn’t. This, from a Toyota dealer, is typical of the online consensus on this topic:
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Will I Get Better Fuel Mileage With Premium Gas?
May 5th, 2017 by Toyota of West County

You pull up to the pump to fill your tank. On the sign, you see premium gas shown as if it’s the best thing since sliced bread. It makes you wonder what it actually does for you. Does it make your car run better? Does it improve your fuel mileage? What is the benefit to using premium gas?

The ‘Premium Gas’ Deception
Instead of referring to it as premium gas, a better term would be hi-octane fuel. The octane rating in premium gas is higher, and all that means is that it is a more stable-burning gasoline. But does that mean it’s better for your car? Only if it’s required.

Does My Toyota Require Premium Gas?
Currently, there aren’t any Toyota models that require hi-octane fuel. The minimum octane rating your vehicle requires is 87 octane, which is standard pump gas or regular fuel. While it’s best to use top-tier fuel from major fuel station chains, you don’t need to use premium gas for your engine to run well.

What If I Use Premium Fuel?
There’s no need to use premium fuel in your Toyota. However, if you choose to do so, there aren’t any negative reactions that will occur. Your engine will operate properly using premium fuel without any concern.

Will I See Improvement in Fuel Mileage?
There’s very little chance that you’ll notice a difference in fuel mileage. If you do notice a difference, it will be negligible at best. Your Toyota is designed to operate at peak efficiency on regular gas, which is 87 octane.
==

Your trucks, your bucks, but objectively, buying premium is a waste.

Forget about higher octane. You are wasting your money.

You'd be better off spending extra for the no ethanol gas, if available in your area. That may make a difference for your engine in the long term.

Pure-gas.org - ethanol-free gasoline in the U.S. and Canada

I use this page a lot for finding Ethanol free. I think the SE region of the country is a little more forgiving allowing us to use it. These stations aren't plentiful, but they are located close enough to me to make it doable.

I have an 03' V8 that I figure considering it's age, it will help the fuel system to run "E" free. Compression ratio is 9.6:1 in these older 2UZs, so I run the rec 90 octane.
Anything more than 90 is kind of a waste.

Now the 5th gen V6 is 10.4:1, but with the Dual VVT-i being able to adjust timing on the fly when the knock sensors kick a warning, it will run fine on 87 or 93.
You may have a little trouble climbing a steep grade at altitude on 87, but for 80% of the 4Runner owners out there, 87 is fine around town or cruising at hwy speeds.

There is a lot of misinformation in here.

Higher octane than your engine calls for is a waste of money.

Lower octane than your engine calls for will run, however your engine will detect it and retard your engine timing resulting in reduced power and fuel economy.

this argument has gone on since the dawn of time...........everything I have read from people in the know automobile manufacturer, gasoline companies etc. etc. using the higher octane if you don't need is a waste of money period........end of story.......

Not worth it. We have retard timing so 87 is just fine. Ran 87 in 2 tacomas (1GRFE) and this current runner. That is, until i went supercharged so now it’s 91 which is necessary.