Re: Most Economical Speed (Long) - Saab 9-5 Bulletin Board - Saabnet.com
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Re: Most Economical Speed (Long)
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Posted by BrooksG (more from BrooksG) on Tue, 24 Sep 2002 09:06:18 Share Post by Email
In Reply to: Most Economical Speed, Mat, Tue, 24 Sep 2002 06:12:35
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I'm operating on memory and it has been a while but IIRC the best volumetric efficiency and the best fuel milage occurs at the max torque RPM. There are two problems with this. First, this assumes full throttle and with the ECU producing a flat spot on the torque curve, there is a question of at which RPM is this really. Since we don't drive at full throttle, other factors come into this. One is the throttle opening, to a certain point, the wider it is open the better the efficiency and another is wind resistance, which increases with the square of speed.

The throttle opening part is something that can be used by the designers if they want the car to get great milage at a certain speed. What they would end up designing for is a car that was geared so long that you required approximate 2/3 of full throttle to maintain that speed. Not fun because the car would be a slug in high gear. The wind resistance is the main milage killer though. The efficiency of the engine does not increase as fast as the wind resistance and consequent drag.

A bit of a ramble but the upshot you don't want to hear. Somebody previoulsy asked on this board for the best milage anybody had gotten. The answers came out at better than 40 for the 4 cylinder engines at 45 miles per hour in 5th gear. In other words, the slower you go in any gear the better the milage gets. A reset of the SID driving around the plant where I work (basically flag ground and a very large building) indicated that my Aero gets 43 or so miles per gallon at 20 MPG in 4th. If you want milage, lug the engine. If you want good throttle response and fun while driving, forget the milage, the two are contradictory.

posted by 4.47.6...

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