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Vman455
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Join Date: Feb 2012
Location: Champaign, IL
Posts: 1,422

Little Red - retired - '05 Honda Civic EX
90 day: 49.03 mpg (US)

Pope Pious the Prius - '13 Toyota Prius Two
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90 day: 55.54 mpg (US)
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Quote:
 Originally Posted by mpg_numbers_guy I've thought of this, but wouldn't the reduced weight reduce coasting ability? Like, when going down even a slight downhill my MPGs jump to like 85...wouldn't the weight reduction require the generation of more momentum in cruising, thus cancelling out the benefit from the lighter weight?
I'm thinking back to my high school physics here, so someone jump in if I get something wrong--several factors would seem to be in play here:

-acceleration due to gravity: acts the same on bodies regardless of mass
-momentum: proportional to mass and velocity
-rolling resistance: proportional to the product of Crr (which increases with mass), velocity, vehicle mass, and gravity

Assuming identical drag area for two cars with different masses, the question is: does the heavier car benefit from its extra mass by the proportional increase in momentum?

The heavier car will be disadvantaged in requiring more force to accelerate to a given velocity and to maintain it (since a car, even at a steady speed, is acted on by acceleration--the twin forces of aerodynamic drag and rolling resistance are constantly acting on it opposite to the direction of travel, requiring acceleration to maintain a given velocity). The heavier car will also be disadvantaged by more losses to rolling resistance while it is coasting or cruising.

So, the question above becomes: Does the increased momentum of the heavier car outweigh the increased rolling resistance and the necessary increase in force (and power) to achieve a given velocity in the first place? My gut says no--you can't get more energy out of the system than you put into it, and you're suffering more losses (reduced efficiency) during the coast or cruise itself because of the increase in rolling resistance; so the increased coasting distance and increased momentum during cruise likely isn't going to offset the increased fuel required to get up to speed and maintain it compared to the lighter car. But, I suppose you'd have to sit down and actually use your vehicle mass, BSFC, etc. and get some real numbers. (If, for instance, the increased mass puts the throttle in a more open position and at a better BSFC to achieve the same acceleration, it might end up being more efficient).
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