If my car needs 91 octane, can I mix 89 and 93 half and half?
Yes.
As I understand it, that’s exactly how the pumps at gas stations work.
Correct and correct.
Just mix the 89 and the 93. Don’t add any Half & Half, that will mess up your fuel injectors.
It’s a pain in the arse though, since you’ll have to swipe your card again in order to use another grade and you’ll often get an error message. I just fill with 93.
Do they actually mix in my tank if I do half a tank of 89 and half of 93? How long should that take?
They should mix pretty well in tank by the mere act of adding one after the other. Gas courses down the fill tube and into the tank at a pretty fair clip, so should do a good job self-stirring. Driving and road vibration, plus self-migration (differing local concentrations mixing together through osmosis) should make a perfectly blended mix within a few hours.
If you’re worried, just cap the filler after adding the second batch, pick up the car, and give it a few brisk shakes. 
That depends on your definition of “half”.
If you buy $5 worth of 89 and $5 worth of 93 and mix them together, then NO, you won’t end up with 91 octane. It’ll be more like 90.8 or something because you got a smaller quantity of the more expensive gas.
But if you buy 2 gallons of 89 and 2 gallons of 93 and mix them together, then YES you will end up with 91 octane.
But it’s probably not true that your car “needs 91 octane”. It’s more likely true that your car would run just fine anything that is almost 91. Ninety might be close enough. But it also depends on the barometric pressure, which is affected by altitude. Your car doesn’t actually NEED 91, but the manufacturer has decided that if you buy 91 octane you’ll have very low risk of pre-ingnition, whether you’re in Denver or Death Valley. At 89 or 90 octane, your results may vary.
If you do most of your driving in the same town, and you want to minimize home much money you’re spending on gas, start by filling your tank half full with 89 and drive around a while. See if you have any knocking or pinging. If so, then add some 93 (a little at a time) until it goes away. Do this over and over until you’re confident you have the right ratio.
Depends on how bumpy the road is, really.
Seriously, though, unless you have a carbureted ancient V8 and buy your baby-blue-dyed leaded gas at the airport, filling up when it’s half-full and alternating 89 and 93 should be acceptable, modern electronically-controlled engines can handle pretty much anything sold as “gasoline” in a civilized country. They may not be happy with it, and certainly won’t make the full advertised power, but they probably won’t throw pistons out the sides of the block.
Edit: 6oz bottles of lead substitute/octane booster work, so splitting the difference with midrange and premium gas has to mix even better.
Also re: the post right before this one: You won’t notice it pinging, modern engines notice it first and adjust the timing so it works.
FWIW, my car calls for 91 octane but my pump choices are 87, 89, and 93. Mixing it is too much trouble for me so I have been putting 89 octane in for 100,000 miles with no issues. Modern high-compression engines have knock (pre-detonation) sensors compensate for lower octane. You might get a little less power with 89.
Probably also (slightly) worse gas mileage. But yeah, 2 points either way probably doesn’t make that much difference. Use 89 most of the time and fill it up with 93 on payday when you remember to, and you should be good unless you live on top of a mountain or something.
With a modern engine, you’re not going to get pre-ignition anyway. What you will get is retarded timing to avoid it, based on the data from the knock/ping sensor. Higher octane gas is not as likely to pre-ignite, so you can run higher compression ratios and more aggressive timing
The 91 octane gas is probably the minimum octane value at which the timing’s not typically retarded, and that you’re getting optimum performance out of your engine. Anything higher than that isn’t going to buy you anything, but it’s not going to hurt anything either… other than your wallet. You could put 87 octane in your car, and it wouldn’t hurt anything, but the engine control computer would likely retard the timing such that you wouldn’t get very good performance out of it.
In your case, I’d just run the 93 octane; you’re not likely to save much money blending 93 and 89 to get something near 91, it’s a pain in the ass, and with 93, you know you have more than the minimum octane for ideal performance.
So, my friend had a car that took super high octane (like 118 Octane). It was ridiculously expensive. Was he just throwing money away?
He was probably just making shit up. 118 octane is racing fuel. Unless the engine was modified for drag racing there’s no reason it would have required it. Even a McLaren P1 will run normally on 93 octane fuel.
In what country? Octane ratings are skewed depending on what country.
98 in Europe is 93 in the USA. Sounds like racing fuel, which would be up near 100 octane in the USA.
We don’t know if he was throwing his money away unless we knew what kind of engine he was using it in, and whether the engine was modded and what its intended use was.
.
Nitpick: 98 RON would be 94 R+M/2.
Deep nitpick: as a physical property, it does not follow that a 1:1 mixture of 89 and 93 octane liquids that include components other than n-heptane and iso-octane would have an octane rating of exactly 91. Of course, any tiny difference would be far too small to matter in practice.
So if you have 118 octane aviation fuel and 68 octane outboard motor fuel, don’t assume you can just mix them 1:1 to get 93 octane automotive fuel.
Since most gas pumps have just one hose these days, wouldn’t there inevitably be some mixing with fuel remaining in the hose?
My car likes 93 octane, as per the manual. I run it on 92 around where I live, seeing as 93 is typically not available at all, with no problems. Since I’m not racing it, I don’t even notice the difference between 93 and 92.
Now, the added 10% ethanol versus 0% ethanol: that’s a whole other story.