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Originally Posted by PackerlandPrius
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I filled up this last tank with Premium. I drive almost exactly the same route and amout every week. I found that I LOST nearly 2 miles mpg. I went from an average of 52.3 mpg the last 3 weeks to 50.1 this last time. The driving conditions were the same and were similar highway and city as with the regular gas.
I'll be sticking with regular.
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Good experiment although a couple of tanks would ordinarily be needed to equalize all variabilities. But it is helpful to review the Atkinson cycle used in our Prius engine.
The Atkinson cycle keeps the intake valve open during part of the compression cycle. This reduces the 'charge' and the compression ratio to 8-to-1. Reducing the compression saves the energy needed to compress the fuel-air mixture as well as reducing the fuel octane requirement. After the fuel-air mixture burns, the hot gases have a 13-to-1 expansion ratio. This provides the power stroke energy.
Bob Wilson