Can I Add 92 Octane Gasoline After Adding 95 Octane?
3 Answers
It is possible to add 92 octane gasoline after using 95 octane. The higher the octane rating, the greater the isooctane content and the stronger the anti-knock performance. Whether it's 92 or 95 octane gasoline, the only difference lies in the isooctane content—there is no distinction in gasoline quality or the calorific value per unit volume of fuel. Standard gasoline consists of isooctane and n-heptane. The primary criterion for selecting gasoline octane rating is the engine's compression ratio, which is also a core energy-saving indicator in modern vehicles. Gasoline is a volatile, flammable hydrocarbon mixture liquid derived from the fractional distillation, cracking, or pyrolysis of petroleum. It primarily serves as fuel for spark-ignited internal combustion engines and mainly includes organic compounds obtained from crude oil distillation and various additives.
My old car had this issue before. Once after filling up with 95, I noticed the tank wasn’t full, so I topped it off with some 92. It drove fine with no issues. The mechanic said occasional mixing isn’t a big deal—it mainly depends on the engine’s compression ratio. For cars with a compression ratio above 10, it’s best to stick to the manufacturer’s recommended grade; mixing could cause knocking—that’s the rattling sound in the engine. For low-compression daily drivers, mixing in a pinch a few times is fine. But long-term mixing can lead to the engine reducing its power to adapt to the lower-grade fuel, which actually increases fuel consumption. I remember one time after mixing for three months straight, the emissions test showed nitrogen oxide levels three times over the limit.
Last week at the gas station, I ran into a young guy asking about this very issue. Mixing 92 and 95 octane is like blending beer with liquor—your engine can handle it briefly, but don't make it a habit. 95 has stronger anti-knock properties, and mixing it with 92 effectively lowers the overall octane rating. While the ECU can adjust ignition timing to compensate, its adjustment range is limited. A mechanic friend of mine once disassembled an engine that had mixed fuels—the piston crowns had star-shaped carbon deposits with cracks, clear signs of prolonged knocking. Honestly, different octane grades also vary in additive formulations, and mixing them puts extra strain on the catalytic converter too.
