Thread: 2000 rpm
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Old 02-25-2009, 06:52 PM   #17 (permalink)
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Originally Posted by shovel View Post
the rule of thumb has always been to gear (axles) and/or shift (manual trans) so your most frequent travel speed coincides with the torque peak RPM of your engine. .
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).
I don't know the ideal answer to this quandry, but I want to point out that there's a difference between each of the scenarios laid out here.

The expedition rule of thumb implies that you predict your desired travel speed and then install suitable differential gears which place your engine speed at peak torque RPM when you are traveling at that road speed.

It doesn't mean you are getting the best mileage the vehicle is capable of, but instead you are getting the best mileage at that speed for that vehicle (presumably) - and it is still only a rule of thumb.

I'd wager you're right about the efficiency scenario you described, where you take an existing vehicle with existing/unchanged gear ratios. It's also likely that the expedition vehicle rule of thumb applies far more closely to the types of engines used in heavy overland trucks, than zippy town cars.
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