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Old 11-16-2010, 05:06 PM   #7 (permalink)
PaleMelanesian
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Join Date: Dec 2007
Location: Texas
Posts: 2,321

PaleCivic (retired) - '96 Honda Civic DX Sedan
90 day: 69.2 mpg (US)

PaleFit - '09 Honda Fit Sport
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90 day: 44.06 mpg (US)
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From the theoretical side, with some VERY quick-and-dirty math:

My car uses
110w = 2x 55w headlight
75w = 3x 25w taillight, including high-mounted
185w extra load

Alternator: 60% efficient (Alternator - Wikipedia, the free encyclopedia)

Lights on = 308w extra load on the engine
308w/ 745 watts in hp = 0.41 hp extra load

Using the calculator, my car needs ~10hp to roll at 55 mph.

0.41 hp is a 4.1% loss of efficiency at that speed.
11% loss at 35 mph
2% loss at 70 mph
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11-mile commute: 100 mpg - - - Tank: 90.2 mpg / 1191 miles
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