Aaron Turpen: What’s The Deal With 85 Octane Gasoline? Safe To Use Or Not?

Debate in the Cowboy State Daily newsroom erupted over whether to use 85 octane gasoline or not. As the automotive writer, I was asked to answer the question...

Aaron Turpen

December 29, 20229 min read

Turpen window sticker 5 25 22 scaled

By Aaron Turpen, automotive writer

Debate erupted in the Cowboy State Daily office over Wyoming’s gasoline. A contingent said that 85 octane gasoline is terrible and should never be used. Another said it’s fine and that they use it all the time. Arguments about how long-lived various people’s vehicles were, what nefarious organizations must be behind the push for higher octane fuels, and vaguely coherent ideas about what the octane rating actually means were abundant.

Holiday parties. They can get hardcore around here.

As the resident transportation expert, I was called upon to come up with something answering this question. And to do it in readable form so you, dear reader, can participate. So here’s what my expertise tells me: it’s not a great idea to use it, but it’s not likely to destroy your vehicle if you do.

But before fully answering, I decided to look into the science and see how much government fuddling has been involved in it. The first will give us a solid background, the other tells us how much that background has been ignored or misconstrued by politicians and bureaucrats.

Let’s start with the basics. Everyone who has taken a small engine repair or auto shop class knows that a gasoline engine requires three things to function: fuel, air, and spark (flame). If you’re missing any of these three things, your engine is not going to run.

If you imagine an engine’s cylinder, it’s composed of a tube into which a rod with a round head sized just exactly to fit the cylinder is set. The rod rises and falls within the cylinder, making the space above it (the “chamber”) larger and smaller. The basic formula is to bring in air, compress it as the rod rises, send in fuel at the right time, then ignite it. The ignition pushes the rod back down, the leftovers from the burning are exhausted, and the cycle repeats. The trick there is the compression part, where the air comes in and gets compressed as the cylinder rises.

Octane ratings aren’t about how much oxygen is in the fuel. They measure the fuel’s stability–or how likely the fuel is to ignite on its own without a spark. The higher the octane rating, the less likely the fuel will auto-ignite. The actual octane number we use is an average of two measurements of fuel stability. The U.S. Energy Information Administration (EIA) sets the standards for octane levels in motor fuels, setting the baseline upon which most state regulatory commissions determine octane levels at the pump. In Wyoming and many high-altitude mountain states, the minimum octane level is 85 whereas the rest of the country is usually 87 or higher. There’s a reason for that.

85 octane fuel is less stable (more likely to auto-ignite in the ignition chamber) than is 87, 91, or higher grades. When fuel ignites itself in a gasoline engine, before spark is timed to create optimal ignition conditions, it’s called “knock.” Engine knock makes the engine run less efficiently and can, over time, cause damage. But altitude changes how likely the fuel is to auto-ignite. Significantly so.

Higher altitude means less air density overall. So air being pushed into the cylinder of the engine isn’t as “thick” and doesn’t compress as much as does air at lower altitudes where there’s more of it to work with.

It’s for this reason that my own testing of fuel economy in vehicles is often skewed below EPA expectations for the vehicle: engines are less efficient at higher altitude. Less air equals less combustion for the same amount of fuel. But it also means that lower octane ratings are acceptable at altitude because the gasoline is less likely to auto-ignite. Cylinder pressures in the engine are lower because there’s less air to compress in there. So a lower resistance to auto-ignition (octane rating) is considered acceptable.

The trouble here comes when we look at the science behind current octane rating recommendations. The newest study regarding octane levels (at the pump) and engine knock was done in 1987. That study found that “modern engines” (aka engines new at the time) were more likely to produce engine knock when using 85 octane fuels than they were with 87 or higher. But not by very much.

That study built on studies that had come before, in the 1960s, on the same subject. Similar studies were done more recently, regarding engine knock, but focused on alternative fuels such as the lower energy fuels derived from plants (specifically ethanol). These studies returned mixed results in terms of how low the octane could be, but it’s worth noting that ethanol actually has a higher octane. What all of them found, however, is that 85 is a bare minimum to use.

Today, carbureted and even naturally aspirated engines are far less common. Now we have fuel injection, turbocharging, and electronic controls that are far more sophisticated than were those of 35 years ago–when fuel injection was relatively new and computer controls were primitive.

Then we get to politicians. The most recent arguments about octane numbers mostly revolved around ethanol in fuels and, of course, government funding of said ethanol’s production. If politicians are going to put their dirty mitts into anything, it’s always going to be about money.

The point here is that most of the arguments weren’t about the validity of the fuel at all. They were, instead, about whether farmers should be paid more to grow corn and then whether the corn used to make ethanol should be further subsidized in its production into a fuel. When “science” was mentioned, it was almost always “science” funded by said ethanol producers. And had little to do with our point here about octane levels and engine knock.

At the more local level, the Wyoming Department of Agriculture is in charge of minimum standards for fuel. The WDA basically just re-uses the American Society for Testing and Materials (ASTM) guidelines for octane ratings. To their credit, the ASTM does review those ratings regularly; the most recent being this year. But those are just standardizations for testing octane levels and have nothing to do with what octane level is most appropriate. Those levels are set by the states. The ASTM publication referenced for that by the WDA is from 1956.

So we’re using an octane rating for high altitude based on something written when cars looked really cool, but fuel injection was brand new (and only in race cars). And two decades before electronic ignition timing was first conceived of. Back when people of automotive fame had names like Harley Earl and Nino Farina. We’re using standards that were derived sixty-six years ago. I mean, it’s cool to collect classic things like cars and guitars from that era. But maybe not fuel standards.

So back to near-2023 today. Will the 85 octane sold at most fuel pumps in Wyoming hurt your car or truck’s engine? Probably not. Should you use it? Probably not.

All vehicles produced in recent memory have fuel injected engines and electronic ignition timing. Those are good things. They improve fuel economy and allow the engine’s computer to adjust for things like low-ignition fuels and knock avoidance. This means your engine probably won’t fall apart from using 85 octane gasoline.

But most engines today are running at higher pressures (or at least higher pressure expectations) and many of them are creating that with turbocharging. If you own a Ford, for example, it’s likely that your owner’s manual tells you to use only 87 or higher octane fuels.

It’s also worth noting that no matter the engine, the highest tow ratings are not measured at altitude and are measured using premium fuels. It’s also worth remembering that using lower octane fuels could void your warranty with many of today’s vehicles, including smaller sport utilities and cars. Or it could at least give the automaker an excuse to refuse to honor a warranty if the problem could in any way be related to engine knock.

To be frank, lower octane fuels have lower ignition in the chamber and thus aren’t as efficient. Using a higher octane fuel means you’ll have better timing response and improved fuel economy as a result. But probably not enough improved MPG to make up for the 22-cent (average in WY, as of this writing) difference in fuel costs when going from 85 to 87. But you will keep your warranty intact. What you’ll avoid is engine knock if you go to altitudes lower than about 4,000 feet.

Altitude is what matters most here. Those who routinely drive at higher altitude (3,500+ feet) and don’t put their engines under heavy load (towing, hauling, racing) and whose engine warranty doesn’t require 87 octane, will be fine with 85.

That means the majority of drivers in Wyoming are OK with 85 octane. If, however, you drive long distances or do heavy work with your gasoline engine vehicle, you will likely benefit from using higher octane options instead. This is because you’ll make your engine more efficient and are less likely to get sluggishness and occasional knock at lower altitudes with those higher octanes.

Ultimately, it’s your choice. But I’d go with mid-grade. One less thing to worry about.

Share this article



Aaron Turpen