Will mixing 85 and 90 octane gas result in 87.5 octane gas?

Dear Car Talk

Dear Car Talk | Mar 01, 1993

Dear Tom and Ray:

Both of my cars recommend 87 octane unleaded gas, which is readily available where I live. But sometimes on the road, "regular unleaded" is 85 octane, "premium unleaded" is 90, and there's no 87 available. I've noticed this especially in parts of Utah and Colorado. Which octane should I use?

RAY: The answer's simple, Gus. Use half and half in those situations.

TOM: Yeah. You may want to throw in some heavy cream, too.

RAY: The gasoline you buy at the pump is a mixture of different octanes to begin with, so mixing 85 octane with 90 octane will work fine. You'll get 87.5 octane.

RAY: The reason the octane is lower in those areas is because they're at high altitudes, where the atmospheric pressure is lower. At lower pressure, you can get by with lower octane, which costs less and causes less pollution. In fact, the yaks that climb the Himalayas only use about13 octane!

TOM: So if you were going to STAY in that area for a while, you could use 85 octane, and your car would run fine. But if you're "on the road," and just passing through, use a mixture. Or, if that's too much trouble, be a big spender and splurge for the 90 octane.

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