On a whim, I flew out to Denver to purchase, fix, and repair a derelict Hyundai Tucson and drive it back to Ohio. Long story short, it didn’t work out, so I spent the week enjoying the Mile-High City. Even though I left empty-handed, I enjoyed my experience exploring the area and its oddities, like this weird 85 octane gas. Confused about whether or not I could use this gas in my Hyundai Accent rental car, I turned to Google and came across a video released a mere two hours before my first refill, from Engineering Explained’s Jason Fenske. Like his other helpful clips, the video explained everything I needed to know and helped me fill up and get on my way.

My naturally aspirated Hyundai Accent rental called for 87 octane, colloquially known as “regular” gas because it’s usually the cheapest and lowest octane gas one can purchase. That’s not the case in Denver or other high-altitude locales, however, where “regular” gas is 85 octane.

Puzzled, I wondered if my Accent rental could accept 85 octane gas. Because Denver is more than a mile above sea level, that means the air is less dense and packs less oxygen. In theory, the lower octane rating should play nice with the lack of pressure and oxygen that generates combustion. Generally, as Fenske explains, it does. The lower pressure at higher altitudes makes knocking (premature cylinder combustion) a lot less likely, even with lower octane gas. Still, I wanted to know for sure, and it was like Fenske knew I was at a gas pump in Denver, discombobulated, ignorant, and anxious, trying to figure out what gas I could put in my car.

Fenske explains that driving around with lower octane gas above sea level is most likely fine. Problems shouldn’t arise until you change elevation or encounter situations with increased load. Your engine’s ECU might be capable of advancing or retarding timing to head off knock, but if your gas is below spec, it may not be able to. Engine damage could occur, and you might void your warranty if the car is new enough. 

Now, as Fenske said, my Accent likely would have been fine if it had 85 octane in there. Modern cars are very good at mitigating problems and heading off issues like knock, but I’d rather be safe than sorry, so I splurged on 87 octane. I would rather pay a few more dollars now than have the rental company insisting I foot the bill for a new engine!