Hello guys. If I mix 5 gallons of 90 octane non ethanol gas, ( recreational gas ) with 5 gallons of 110 octane race gas, what does the octane rating come out to? Would it be 100 octane or something other. Is there a formula for such a thing. Thanks.

Not sure on the rating or a formula to calculate it. My question would be...what are you using it for/in?

It’s linear, so in the OP’s example the result would be 100 octane.

Well if it works like it does at the pump there are two methods of obtaining octane numbers, RON or Research Octane Number (usually a higher octane number) and then there's MON or Motor Octane Number (usually a lower octane number). So here in the USA you'll see on the side or somewhere on the gas pump (R+M/2) ( RON + MON divided by 2) and if we use that for your 110+90/2 we would get 100 octane as you already figured you would get

(oops didn't see your post, tree'd

Check it out, I forgot that Sunoco and their "BLEND" pumps had NINE octanes to choose from and all blended from probably two or three in the ground tanks. YIKES. The only thing that was bad about the blend gas pumps was if you rode a motorcycle with a small gas tank like I had back in the mid-'60's like on my Honda Super 90 and just wanted to top it off. You had to get lucky and hope that the pump showed the previous sale and that it was at the least the premium blend. Still to this day there's about two to two and half quarts (or more) of previous sale gas left in the hose to the tank in the ground so if someone pumped the lowest octane that's what you get for the first half gallon or so. Anyway check it out NINE octane level fuels to choose from and the first two quarts are a crapshoot

As others have posted, you can roughly just blend all these confused numbers RON, MON, AKI.

But the important consideration is:
what fuel rating do I need to keep the worst cylinder of my engine from detonating and damaging parts so badly that the engine must be rebuilt, or even replaced?

In the past, were there any groups who had an urgent, life or death, worry about what fuel rating to use?

Yes, the USA Army and Navy during the years when they were using gasoline like fuels.

They rejected RON and MON.

They researched the challenge and came up with the
Army/Navy Performance Number
commonly shortened to
AN-PN.

Gasoline powered aircraft fuel providers still give customers this number:

https://warteraviation.com/fuels/avgas115-145/

AN-PN is far more scientific and should have been used instead of Antiknock Index on USA gasoline pumps.
AKI is one of the best examples of how bad decisions are made in Washington DC.

This thread is wandering, which is to be expected, so let me wander….
Think about this. Ethanol raises octane, so the cheapest way to create “premium” is to simply add more ethanol up to the allowable max (10 percent my state, though I seriously doubt that’s a spec that’s monitored to any degree).
So when we need/buy premium, we might actually be getting more corn juice than the cheaper regular gas.

I'm using it in my 69 Charger R/T 440 motor that has around 10 to 1 compression.

I worked at a Sunoco station in late 60s, had two tanks 260 and 190, mixed with ratio by the blend number. BTW 260 was $.32 a gallon, one penny between each. I pumped all the gas no one touched that hose. Had one guy get $1 of 190 everyday then the hot cars got 260 fill up, sometimes TWICE on Saturday nite. True gas guzzlers lol.

But in that scenario the "premium" should be cheaper. Could we be getting ripped off!?
The Sheetz stores around here offer 88 octane cheaper than 87 because it's E25 or something.

Dunno. I use EF90 like the OP with an additive. Gunna try blending with race fuel soon.