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Most engines that call for 91 can run on lower octane. What people don't seem to realize is those engines will have their fuel and ignition maps adjust accordingly and start running much richer to compensate for the lower octane fuel thus causing you to have a worse fuel economy, less power and more prone to knock. As a result you likely pay the same amount for the low octane fuel as the higher octane since you use more but you rob your vehicle of performance while potentially damaging the engine and reducing the life expectancy.
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