BUT, that 2.5 hp would be stolen all of the time. I would say that idling would be just as impacted as highway mileage. I don't know the exact hp figures, but when my car is idling, the engine load is 10-17% compared to 50-70% at highway cruise.
Then again, I think a lot of people here shut off their engines rather than idle, so that might not have as great of an impact.
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