Pick any production car that is factory-tuned for high octane fuel.
We know that on lower-octane fuel it has to retard timing to avoid knocking. In effect, it has to run at a lower compression ratio. This is practically the definition of reduced fuel efficiency.
My question is: In practice, how much can low-octane fuel affect fuel economy? E.g., if a car is tuned for 93 octane and is given 87 octane, how much worse can the mileage be? 1%? 10%? If it's being run full-throttle/full-load on a track can it reach 50%? I'm hoping for quantitative or authoritative references applicable to modern fuel-injected production vehicles.
