Premium versus Regular

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As I’m sure others will bring up, altitude also affects octane requirements of your engine. At a higher altitude, air is thinner, so you pack less of it into your cylinders, effectively resulting in a lower compression ratio. As a result, your engine’s octane requirement is lower at higher elevations. That’s why regular is a lower octane in Colorado, and your car should do just fine on it, without making the oil moguls that much richer. (Disclaimer: May not be true if you have a turbocharged or supercharged engine).