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Old 11-21-2008, 06:03 PM   #5 (permalink)
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Originally Posted by Christ View Post
I'm not saying that changing gears won't improve your efficiency, I'm just saying that straying too far from your max torque will.
There's obviously a point past which you are gearing "too tall" for best FE, but I don't think that the RPM you make max torque at is really a determining factor.

Figure that a given car needs 10 HP to go at cruising speed on the freeway. To get the best FE, you need to get the engine to make 10 HP as efficiently as it can, and select the gears to match that speed with whichever RPM is the most efficient for 10 HP. I'm pretty sure that is significantly less than where the torque peak is in almost all applications...

Of course, being able to make 10 HP ultra-efficiently won't help a lot if we can't even accelerate to cruising speed, so we need the flexibility to generate more power in a pretty efficient way, with the appropriate gearing to get us up to speed. Thankfully we have more than one gear in the transmission...

Now, according to experienced people who know how to use ScanGauges, the best way to accelerate to a given (steady state) speed is low and slow, and to up-shift somewhere around 2000 RPM. That number is reasonably close in most cars, from Accords to Yarises to Fits to Camrys, and yes Civics too. I doubt that most of those make peak torque that close to 2,000 RPM...

What I'm trying to say is that, while where the peak torque occurs does affect what kind of gearing you want for maximum FE, there are other factors as well. And I think the peak torque RPM is probably not the largest contributor...

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