Thread: 2000 rpm
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Old 02-25-2009, 06:47 PM   #16 (permalink)
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Originally Posted by MetroMPG View Post
Graph a speed vs. MPG, or RPM vs MPG in top gear for your vehicle and I bet you a donut you'll find your best efficiency is far lower down the tach than peak torque (and also below the best BSFC zone).

My CRX Si has short-short gearing, and is only going 40 MPH at 2000 RPM in top gear. At 60 MPH, it's spinning 3000 RPM (yes, it's a linear relationship--it has to be).

The torque peak for my motor is in the 4000-4500 RPM range. I guarantee you that the mileage is lower at 80 MPH than it is at 40 MPH. In fact, I gained about 10% fuel economy by backing off my top RPMs from 2500 down to 2000. This is measured over several full tanks of fuel, since I don't have instrumentation to give me instantaneous fuel usage.

I do know that a steady-state 80 MPH cruise will net me right around 33 MPG on the SF-LA roundtrip. I lack the patience (and the opportunity!) to try it at 40 MPH, but I am pulling right around 40 MPG in my daily commute these days. And since it takes four times the energy to push my car through the air at 80 MPH as it does at 40 MPH, I'd be really surprised if a steady 40 MPH didn't pull down MPGs in the mid-40s or higher.

...BTW, I'm surprised at the OP's Taco takes 40 HP to run at 55 MPH; I would have SWAGged (Scientific Wild-*ss Guess) it to be half of that.

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