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Old 09-18-2012, 06:14 AM   #1 (permalink)
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MPG Tool Math Help

First posting for me here ... just found this site/forum and must say it scores a wow and then some!!

I need help understanding the math in the aero-rolling resistance tool.

The conversion from Watts to MPG has me baffled.

I understand the need to use consistent units of meters, kg, and seconds in values for weight (mass), density, velocity, and area.

I can duplicate the force values in Newtons for rolling and aero and multiply by the velocity in meters/sec to get Watts.

I divide the Watts by 746 to duplicate the horse power in the chart.

Knowing the gallons per hour and the distance per hour makes MPG simple ...

However, that gallons per hour calculation using the fuel energy density and Watts has me totally confused. I am using 100% engine and drive train efficiency and zero parasitic losses to keep things as simple as possible until I understand the math from Watts to gallons of fuel. Seems like I should just multiply Watts by 3600 seconds/hr and divide by the fuel energy density in Wh/gal ... but I get crazy values doing so.

All help much appreciated.

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