Q: RPMs fall when using higher octane fuel, 2005 Hyundai Elantra

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I just refueled my engine with 91 Octane fuel today. It sounds much different and in my opinion better and also feels as if it has much more "pickup" when I stomp on the gas pedal at low speeds, but after 80 MPH on the interstate with the pedal to the floor, the RPMs begin to slowly fall. Is this possibly the higher octane fuel? Maybe spark plugs? Any ideas?

My car has 69700 miles.
My car has an automatic transmission.

As you may know, the compression ratio of your engine determines the octane rating of the gas you must use in the car. Higher octane fuel prevents the air-fuel mixture from igniting before the spark plug does it. Firing the air-fuel mixture at the proper time gives you the maximum power your engine was designed to get. What may be happening when you are giving the engine more fuel by peddling it to the floor is providing the engine with more fuel than it is able to burn at higher speeds. This is a result of the car’s ignition timing being designed to use 87 octane fuel. When you provide more fuel to be ignited given a certain ignition timing, this can cause a slight delay in the ignition of the fuel resulting in the drop in performance as you have described. For more assistance determining the best fuel for your vehicle, YourMechanic has trained technicians who can travel to you and assist in any repairs that may be needed.

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