This thread can be debated forever. If RAM recommends 89 but allows 87, then feel free to run 87. If in the mts, pulling hard, towing, giving it a lot of throttle, maybe choose 89? If driving like me, or on flatter terrain, towing nothing and having an empty bed, than 87 is fine. I draw the line at 87, won't use 85...
A station down the road has E-15 88 octane and E-10 87 octane. Now I'm no chemist and that choice does confuse me.
I do use Costco fuel at times and happily put in 91 in order to allow a mix of the next tank with 87.
I ran 87 on the highway this last weekend and got about 21 mpg, then I put 89 in on the way back and got... 21. So who knows?
Sorry in advance for the wall of text that follows...
I think I can help clear the confusion up on how ethanol content affects octane rating. It's important to keep in mind that the number you see on the pump is an octane
rating and not a measure of actual octane content. An octane rating is a standardized measurement of a specific fuel formulation's resistance to pre-ignition or detonation. It's possible to have no octane in a fuel and still have an octane rating because the blend of that specific fuel has an ignition characteristic that is measured in comparison to the standardized octane characteristic. Straight ethanol, for example, has an octane rating of 113 with no actual octane content. This is because ethanol is inherently harder to ignite than traditional gas.
As you up the ratio of ethanol versus straight gas in a blended fuel, the octane rating likewise goes up. Standard 85 octane rated gas mixed into a 10% ethanol blend becomes the 87 octane rated fuel most of us see at the pumps. Upping the percentage of ethanol to 15% is the 88 octane rated fuel you are referring to. E85 typically doesn't have an octane rating listed on the pump. The 85 refers to the traditional 85% ethanol content although that percentage can vary to as low as 51% because of issues starting engines in cold weather with the higher ethanol content. Fuel that is 85% ethanol will have an octane rating of 108 which is significantly higher than traditional formulations of 91+ premium gas.
Octane itself has no effect on power. More or less octane does not correlate to more or less horsepower a fuel can make in an engine. Similarly, an octane rating has no correlation to how much energy is available in a given volume of fuel. The only thing octane does is make a fuel harder to ignite and likewise an octane rating is only a reference to how hard that particular fuel is to ignite.
That begs the question, why would we want to make our fuel harder to ignite? Wouldn't we want it to burn more easily? While it's true a fuel that's difficult to burn isn't much use in an engine, having fuel that too easily burns or ignites interferes with the normal operation of a traditional 4 cycle internal combustion engine. Physics tells us that the piston in an engine does work on the fuel/air mixture during the compression stroke. The properties of thermodynamics tell us that pressure, volume, and temperature are related for a constant amount of a gas (state of matter in this case, not limited to specifically gasoline fuel). As the piston compresses, the volume the gas can occupy in the cylinder decreases. This necessarily increases the pressure of the gas. Because work is being done on the gas by the piston, the increase in energy from compression is stored in the gas in the form of an increase in temperature. You've experienced this same principle if you've ever noticed how air compressed in a compressor is warmer than the ambient air temperature (at least until the excess heat is transferred to the environment).
Every compression stroke of every cylinder causes an increase in the thermal energy of the fuel/air mixture inside the cylinder. For a standard fuel in a standard engine, the fuel is resistant enough to ignition that it can tolerate the increase in temperature and won't ignite prior to the spark. This is also why high-compression and/or boosted engines typically require a higher octane rated fuel. The more aggressive compression raises the temperature higher than a standard engine and can push the thermal energy of the fuel past the point of ignition prior to the spark. This can happen before the compression stroke finishes and can be destructive to the engine because the piston is being pushed up by the crankshaft and down by the ignited and rapidly expanding fuel/air mixture before crossing TDC.
On the other side of the cycle past the point of compression and spark, a separate but related phenomena can occur. An ICE
really wants the spark to be the only source of ignition in the fuel. After reaching full compression, a spark is created and the heat of the spark ignites the fuel closest to the spark plug. As the burn propagates outward from that point through the fuel, force is evenly and smoothly exerted on the piston which drives it back down the cylinder and drives the rotation of the crankshaft. Hot spots in the cylinder wall have the potential to ignite the fuel touching it independently of the burn radiating from the spark plug. This separate detonation exerts a force that at least partially opposes the force of the desired burn. This is what people refer to when they say an engine is "knocking". Modern engines have sensors that detect this and will retard the engine timing to reduce the likelihood that detonation will happen. If the detonation is severe enough and exceeds the engine's ability to compensate, engine damage is likely.
The octane rating of fuel needed for an engine is determined by the engineers that design it. They calculate what the normal operating conditions of the engine are and what environment the fuel will need to tolerate without unwanted ignition. In our case, the engineers have specified the engine to run as designed on a fuel with an octane rating of 89 or greater. All this means is that a fuel with that octane rating will be sufficiently resistant to ignition to not cause detonation or pre-ignition under any normal operating condition.
So what about the manual saying 87 is all right but not optimal? That just means that the engineers determined that engine knock is possible with this fuel but the engine is designed to be able to compensate for it should it happen. Conversely, they determined that 85 poses a higher risk of knock or pre-ignition severe enough that it can't be compensated for and could cause damage.
This is where everything gets more muddled and why you'll see some people say they see no difference between 87 and 89 and why some people swear it makes a huge difference. If you're running 89, you're going to be on a pretty even standing with anyone else running 89. Under normal running conditions of the engine, 89 octane rated fuel should be able to tolerate any environment that exists inside the cylinder without igniting outside of the spark. But a lower octane rated fuel does not mean lower power or lower energy. There is just as much chemical energy in 87 as there is in 89. If your specific engine and environment and driving style allows 87 to run without knocking, the engine will not retard timing and you will suffer no computer-induced power or economy loss. It will run no different than it would with 89. However, if you do experience engine knock with 87, your engine will compensate and ultimately power output will be reduced. It's important to note that this reduction in power is the result of your engine artificially limiting itself and not because the 87 octane rated fuel isn't capable of producing the same power.
Now to throw ethanol back in the mix, why wouldn't we want ethanol blends that increase the octane rating of our fuel? Wouldn't we benefit from getting away from the 87-89 questionable zone and use ethanol to further increase the octane rating to where knock and pre-ignition would never be a problem? The answer is the same reason you will always have some level of reduced performance and/or mileage with an ethanol blend.
For the same volume of liquid fuel, ethanol releases less energy than traditional gasoline through combustion. This equates to making less power with the same amount of fuel and likewise needing a larger volume of fuel to make the same power. How noticeable the difference between straight gas and a 10% ethanol blend is depends entirely on how the engine is used and the environment it is used in. For example, city driving with lots of starts and acceleration will have a more noticeable fuel economy hit than cruising at low RPMs down a highway. The result of this is some people see significant power or economy improvements switching from ethanol blend to straight gas while others will have a negligible increase.
This all results in a simple answer to the question of what fuel should you run....it depends. The specific way in which we each use our trucks will determine what is most economical for us individually.
Let me share my experience with my last truck for example. I drove a 2016 Silverado LT with a 5.3L V8 for 40 months and 44,000 miles before trading it in on my new Ram. My driving consists almost entirely of a daily 80 mile total commute to work primarily on an interstate. It was flex fuel and I did a fair bit of testing with every fuel type I had available. Where I am, E85 is the cheapest fuel. That is followed by 88 octane rated E15 and 87 rated E10. Straight gas is more expensive and available as 87, 89, and 91/93. With E85, I would get anywhere from 12-14 mpg depending on seasonal formulation. E15 was about $0.15 per gallon more and I would get 16-17 mpg. E10 was an additional $0.10 per gallon and I would get 17-19 mpg. Because of local ethanol subsidies, straight gas is significantly more expensive. While I could get 19-20 mpg with any octane rated straight gas, the increase in cost made it less economical to run over E10.
So far in my Ram, I'm at a little over 2200 miles and I've run a few tanks of 88 rated E15 and 87 rated E10. E15 has given me around 15 mpg and E10 has been closer to between 16 and 17 mpg. There's not a ton of 89 around here anymore so I'll likely run a few tanks of 91 or 93 rated straight gas to see if my gains are enough to justify an extra $0.50 per gallon. I doubt they will be but data points are data points.
If you are more concerned with always getting the most power out of your engine, run 89 rated straight gas and call it a day. If you're like me and want the most economical fuel, find an app on your phone to track mileage and start trying different types of gas. Everything else being equal, straight gas with an 89 octane rating will get the best mileage and allow the engine to make peak power. Higher octane straight gas will give the same mileage and make the same power while being more expensive. Straight 87 rated gas will potentially (but not necessarily depending on conditions) reduce power and mileage but is still safe to use according to the manufacturer. As you increase the ethanol content in blended fuels, price will go down as well as mileage and peak power. Local gas prices and availability will dictate what is most economical for you to run in your truck.
