I ride an antique motorcycle so I can't buy any of them... But we'll get to that.
First things first, what is the difference between 87, 89, and 91? What do those numbers even mean? and why are there all these new ones?
The numbers are called "Octane Ratings" which is where we get the phrase "high octane" but what does that measure and why is having more a good thing? In technical terms, the number represents the amount of heat required to ignite the fuel, as expressed in a percentage related to isooctane where heptane is 0. But that probably doesn't help you unless you already know a lot about petroleum products. So I'll start with some common misconceptions and work up to a more understandable explanation.
Octane ratings are NOT a measure of how much energy (or power) is in the fuel.
And while they are a measure of how combustible it is, Higher numbers actually mean the fuel is more difficult to ignite. If you held a match up to the different fuels, the 87 would ignite most easily, but realistically... you wouldn't notice much difference. They would all burn about the same. It's not like we're talking about jet fuel here! (but again, we'll get to that)
To understand the importance of the numbers and what they mean, first you have to understand why faster cars need a higher number. In the beginning, there was just "gasoline" and it was all about the same. But as engineers started to tune for more and more power they started to run into problems. Lots of problems... But the one we are interested in is called "Detonation" or "ping". Fuel detonation might not sound like a bad thing. It's supposed to explode right? But it's more accurately called "Early" or "Spontaneous" Detonation. When things are going well, the spark plug ignites the fuel, and the expanding gas created by that explosion pushes the piston down to create power, almost like pushing the pedal of a bicycle. But when the engine gets too hot, or the pressure inside the engine gets too high, (both problems that crop up when trying to make more power) the fuel can Spontanioulsy ignite itself. When that happens, you have TWO explosions happening at the same time, in the same small sealed compartment... and that isn't good. What happens is that the pressure shockwaves smash into each other and combine to make an enormous amount of heat and pressure. This can cause massive stress and damage to the engine, and can even blow holes right through the pistons. And that gets expensive to fix.
Over time, engines have become more and more high-tech, and people have come to expect more performance from less fuel. The way that is possible is by designing engines that create more pressure from smaller doses of Gasoline. General design changes account for most of it, but then you add something like a Turbo or a Supercharger and the pressure in the engine increases dramatically. High pressure means more heat. High heat and High pressure cause Detonation, and that can lead to expensive engine replacements!
And that's where the High Octane fuels come into play. 87 Octane takes 87% as much more heat to ignite as isooctane does compared to Heptane. Or, in other words, it takes 87 Octane units of heat to ignite it. 93, takes MORE heat to ignite. So you can use it in higher performance engines like ones with Turbochargers without the increased heat and pressure causing Detonation.
Put very simply, the higher the number, the more heat and pressure the fuel can withstand without going off by accident!
So what does this mean for the average consumer? It means that there is practically NO DIFFERENCE between the three grades of fuel unless your engine was designed to run on the expensive stuff. Faster, higher-end cars are likely to be tuned for better performance over cost savings so many of them require the better fuel. But if your car doesn't require it, it's a waste of money. Putting the expensive stuff in your corolla will not make it more powerful, more efficient, or more reliable unless something else is seriously wrong. Putting the cheap stuff in a car that is designed for Premium can cause enormous damage, but luckily most modern cars have electronic sensors that will notice and change the tuning to reduce power output and prevent any damage to the engine. So you CAN use it in a pinch, but it's definitely not a good idea.
Something like 10% of cars require premium, maybe 1% require the mid-grade, and the rest see no benefits from the higher grades whatsoever. But, sales break down at about 5% for mid-grade and about 10% for premium. This suggests that a lot of people are putting midgrade gas in their regular cars, and studies have estimated that costs consumers over 2 billion dollars a year nationwide! Not to mention... the majority of stations only have two tanks. "mid-grade" is really just regular with a bit of premium mixed in. There is a very small number of cars that require this, but in general, you usually end up paying closer to premium rates for mostly regular gas. And that's before you consider the increased environmental impact of burning the premium stuff for no reason.
But if that wasn't complicated enough... Now we have Ethanol. Ethanol isn't gasoline at all. It's alcohol. It's actually exactly the same alcohol we drink which is probably the only reason we haven't seen any vehicles designed to run on 100%. Imagine a gas pump that dispensed the best moonshine imaginable for less than the price of cheap gasoline. Sounds dangerous to me. It's a shame we can't all be responsible and cut our dependence on foreign oil but realistically, I don't see that happening any time soon. Not to mention, the oil companies and the alcoholic beverage companies can't have people finding out we can grow and produce this stuff renewably for like $2 a gallon and get both for one low price.
But what's the difference? Can we really just fill our cars with moonshine to save money? That sounds like a bad idea you're crazy uncle told you about.... But the short answer is yes. They work about the same. Minor tuning adjustments and it would work great in the cars we already have. A lot of them wouldn't even need a tune-up. The bigger problem with Ethanol is that it dissolves rubber. Fuel lines are made of rubber, and in a modern car they can be pressurized to well over 100PSI! (a regular tire is about 30PSI) Dissolving your fuel lines is NOT a good idea. So most pumps dispense about 10% ethanol and 90% gasoline. This keeps the Ethanol diluted enough that it doesn't cause a problem.
But newer cars have been designed with Ethanol in mind. They use fuel lines built out of more resistant stuff that doesn't dissolve. This means they can run the E15 (15% ethanol), E30 (30% ethanol), or even E85! (170 proof corn liquor with some gasoline mixed in for taste!)
The benefits of Ethanol are HUGE! For one, it's cheaper and it literally grows on trees. (or corn stalks) But it also burns cleaner and has an octane rating of 113!!! That's why the regular jumped up from 85 to 87 when they started mixing in some Ethanol. 91 became 93, and a lot of racecars require E85 these days. There really isn't a downside if you have fuel lines that can deliver it and enough self-control to avoid a cheap sip of shine while you're filling up.
So then... why do some crazies resist? Why are some people so angry about progress that they insist on paying more for Ethanol Free? Well, that one's easy. My carburetters have rubber parts inside of them. Ethanol dissolves rubber. The rubber diaphragms are very thin, cost $150 each and there are 4 of them. Under normal circumstances the fuel doesn't come into contact with them... but that's a $600 risk I just don't want to take. And that's if you can even find new ones. But it's not just people with old cars, old carbs, and old rubber gas lines. There's another group who buys it and they are even angrier than we are because they use it to do actual work. A lot of small engines aren't as sophisticated and aren't able to mix the fuel and air together as well as a regular car. Most lawnmowers, weed whackers, and small equipment struggle to burn ethanol, and a lot of people have really expensive ones they use to make their living. Sure, they could upgrade but... that hurts profits and they probably like the old reliable ones better anyway.
You'd think we'd get along but my bike is modified to have a higher compression ratio for more power so I want 97 Octane and I'm happy to pay $4.50 a gallon when I can find it.
Their lawnmowers usually aren't modified for horsepower... So they would rather buy the 85 for as cheap as possible and keep more of their profits.
But this is getting to be an issue for the gas stations. That's 2 tanks for regular gas, A tank for Ethanol, 2 tanks for Ethanol-free, and someone is still going to show up and ask for Flex-fuel. (Flex fuel is pretty much gasoline sausage. It's something like 50-70% ethanol, mixed with regular gasoline and it's cheap because it can vary so they just use whatever they have leftover, but those cars can use all of the above.)
Most stations don't have 6 tanks! And that's before you mention Diesel which they make a lot of money off. So that leaves them in a position where they have to choose. A lot of stations were using all their tanks before things got weird so they have 3 grades of regular gas and that's it. I don't think I've ever seen more than 5 on one pump, so you end up seeing weird things like 85 (ethanol-free), 88 for midgrade, and then regular 93 for premium which makes the lawncare guys happy, but it's no good for me and it means the regular is going to be more expensive than the competition. Or, they go the other way and you see 87, 89, and then 93 (ethanol-free) which is great for me and great for regular cars, but the landscaping guys get scalped and the people who already don't want to pay for Premium have to pay more than usual.
Some stations get new tanks put in for new fuels, or have one around the back for something other than the normal 3. I envision a world where the pumps are sophisticated enough to mix any combination on the fly so they could fill the three tanks they have with Low octane ethanol-free, High octane ethanol-free, and pure Ethanol to make any of the 8 options on the fly. But, the oil companies aren't going to let that happen because they like watering down the tankers with 10% Ethanol before they send them out to save money on their end, they like to stay in control of the ratios and the pricing, and most importantly... People would be outraged if they found out a gallon of pure, drinkable, 200 proof ethanol actually has a retail value of about $2 but they are paying $4 for oil-based petroleum and 15 to 20 bucks for a bottle of 70 proof vodka that's 60% water.
So the stations pick what works for them and their customers. And a lot of them have invested in more tanks and new pumps to get people what they are looking for at competitive prices.
So just to recap, we have covered 9 different products that you might fill your tank up with and haven't even started on diesel. There's Regular , Plus  which is mostly a scam, Premium  which has a higher octane rating meaning that it can withstand more heat in a high-performance engine without exploding too early, E15, and E30 which are Regular gas mixed with moonshine to save money, E85 which is dirt cheap and has a SUPER high octane rating but only works in cars that were specifically designed for it, Low Test Ethanol Free - which the landscapers buy to run their lawnmowers, High Test Ethanol-Free - which is just good old fashioned high-quality gasoline for people who are stuck in the past, And Flex Fuel, which is pretty much just a random mix of whatever they had leftover when they made the rest of those.
Add it all up and that was about 123.73 billion gallons of fuel in 2020. Which was actually 14% less than 2018 and the least we have used since 1997! (mostly because of covid)
But then there was the 44.61 billion gallons of diesel and over 10 billion gallons of Jet fuel (WAY down from the 18 billion in 2019) If my math is correct, that would fill enough milk jugs placed side by side to wrap around the earth 673 and a half times! That's a lot of fuel! But it's 4 am so don't quote that on your environmental studies.
Diesel doesn't have an octane rating. Diesel is actually sort of a byproduct of gasoline production, it's made up of some of the heavier less-flammable parts that are left over after gasoline is refined out of crude oil. Unlike gasoline, Diesel will not burn when exposed to an open flame. It must be exposed to extreme heat and pressure to cause an explosion. Because of this, Diesel would have an octane rating so high it would not be a relevant measurement. Instead, Diesel has a Cetane Number. Cetane is a chemical compound found naturally in diesel that ignites easily under pressure. Because of its high flammability, it serves as the industry-standard measure for evaluating fuel combustion quality.
A Cetane Number is a measure of the ignition value of a diesel fuel that represents the percentage by volume of cetane in a mixture of liquid methylnaphthalene that gives the same ignition lag as the oil being tested or in other words, it's a measurement of how much Cetane the fuel contains which gives an indication of how well it will ignite and burn in an engine. The scale for measuring cetane ranges from 0 to 100, and the higher the number, the better. Modern highway diesel engines tend to require fuel with a cetane number ranging from 45 to 55. Typically, Regular No. 2 Diesel has a cetane number of 40 to 42, while Premium Diesel has a cetane number of 47 to 52.
It always says "Low Sulfer" but Low Sulfer Diesel fuel is sort of an oxymoron. In 2001, the EPA finalized a federally mandated program called the 2007 Heavy-Duty Highway Diesel Program. This program was established to further decrease emissions by enabling the use of advanced emission control technologies for new highway diesel engines. Although effective, these technologies were found to be easily damaged by sulfur, requiring serious sulfur reductions in diesel fuel in order for them to be used. Effective June 2006, the maximum sulfur limit in diesel was slashed from 500 to 15 parts per million (ppm). Low Sulfer really just means "EPA approved for highway use".
The last type of fuel you will usually see at the pumps is Off-Road Diesel, and while some guys might be tempted to put this in their pickups to make them even more rugged, it really isn't a good idea. The only real difference between the two is that Off-Road Diesel is dyed red so that the cops can easily check your tank and make sure you bought the stuff that includes road taxes. It's literally the same stuff, but you get a better rate if you are using it for a tractor or something because the price of on-road diesel includes taxes used to maintain the roads and bridges.
DEF (often seen for sale alongside highway Diesel) is not Fuel at all. It's Diesel Exhaust Fluid. This is a solution of 32.5% urea in 67.5% de-ionized water. DEF is clear and colorless and looks exactly like water. It has a slight smell of ammonia, similar to some home cleaning agents. DEF is used by Selective Catalytic Reduction (SCR) technology to remove harmful NOx emissions from diesel engines. These systems are required for further emission control on all large Diesel highway vehicles produced after 2010.
Oh, but there is one more thing you might see at some stations. Kerosene! In America this is usually used to run heaters, but in a lot of countries it is still used in lanterns which are the main source of lighting in many homes. I use it to clean my motorcycle chain every 300 miles, and there are countless other uses for it too. One particular use that might shock you is that kerosene is often known by another far more exciting name... Jet Fuel! They are essentially the same thing. Throw a match in a bucket of either one and the match will go out. Jet fuel isn't half as exciting as Hollywood seems to think it is.
On occasion, you might see a station that still has a Water pump. Those are for filling radiators, but most modern radiators are pretty good at staying full all on their own so most of the water pumps have been phased out or replaced by pressurized air for filling tires.
And of course, that Nasty Bean Juice they sell inside. Some freaks actually fuel THEMSELVES with this stuff! But me, I try to avoid anything that addictive... It's literally just water they heat up and drip over some fancy imported dirt to make some knock-off Columbian bam bam. And it tastes like it. But some fools pay like $16-$20 a GALLON for that crap so most places have it on tap. If you thought 2 billion was a lot to waste on Plus grade gasoline, just think what we could do with the 80 billion Americans spend on Coffee every year. I'll save my money for that 93 Octane ethanol-free gas. It's a lot cheaper and a lot more fun.
So there you have it. The 14 different fuels that keep America moving. What the numbers mean, what they have in common and why they go in different tanks. But the most important thing you need to remember is that the vast majority of people buying "plus" grade gasoline are just throwing billions of dollars away for absolutely no reason. Don't do it, unless you are one of the few people whose car actually requires it. You should buy the LOWEST octane rating that isn't less than your vehicle requires. If it doesn't require fancy gas, buying the expensive one won't make it any faster.