My wife drove my Escape on a road trip of 1,800 miles. She grumped about putting "High-test" in it, so I had her try two tanks of mid-grade. To our surprise, the MPG dropped so much the "savings" was offset by the cost-per-mile. I forget the exact numbers, but 93 went farther on a tank than 87, easily offsetting the "extra" cost. The rest of the trip was done with 93 (or 91 where there was no 93). The fuel-cost-per-mile was much better with the higher-octane fuel. The extra power had value, too.
My wife considers a car just a "refrigerator on wheels" (just an appliance). She noticed right away her cost was cheaper with the "more expensive" fuel. The return trip (with me at the helm) was even better. Our best tank was nearly 40% better than anything on the outbound leg. Best tank: 29.3 mpg, average nearly 25 mpg. That was exclusively on the highest available octane rating. The return trip needed one fewer tank to get us home.
Since higher octane produces more power (when the computer can deal with it) and it takes nearly a fixed number of ponies to offset all the barriers to power, the engine uses less fuel to produce the same result. If the motor gives 300 hp on 93 and 240 on 87, it's working more easily when it can maintain desired speed on less fuel-per-mile.
