Thread: 2000 rpm
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Old 02-25-2009, 11:48 PM   #18 (permalink)
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Originally Posted by MetroMPG View Post
Cruising at peak torque RPM for best fuel efficiency is a "rule of thumb" myth that gets regurgitated regularly. Not sure why.
I notice that as well and I think the mix up is mistaking the engine BSFC islands for the vehicle optimum FE.

There is one general rule of thumb which states:

"As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s).
Piston speed (ft/s)= 2*stroke(inches)*rpm/720.

Piston speed = 2 x Stroke in inches x rpm / 720"

As with any "rule of thumb" there are as many exceptions to it as cases supporting it.

In my car GM 3.8 V6 with auto 4L60E trans and four door sedan body 2 000 RPM = 110 kph (65 mph approx) and the most economical steady cruising speed is around 1750 RPM (95 kph = 55 mph aprox.)

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