Ever Wonder how an aero change will affect your mileage?

Ok, you don't have a Wind Tunnel handy, and you want to know if adding that Roof Rack, Bike Rack, Mud Flaps, or even Synth Oil are going to affect you.

Here's an emperical way to calculate just how much extra oomph you're going to require to haul your goodies.

I admit, I stole the words http://realbig.com/detomaso/1997-06/336.html

The concept is called "Coast Down Horse Power"

CDHP = (WEIGHT*MPH)/(823.3*CDTIME)

Where:

CDTIME = coast down time in seconds

CDHP = coast down horsepower (i.e. the horsepower required to maintain a given speed.

This formula can be used in determining the effects of changes made to a vehicle to alter its aerodynamic drag or rolling resistance. As an example, assume you have taken coast down measurements from 65 to 55 mph (under similar atmospheric conditions) before and after making changes to reduce aerodynamic drag (e.g. lowered the vehicle and added an air dam). In the before case, it takes 15 seconds to coast down. In the after case it takes 20 seconds. Assume the vehicle ways 3400 lbs. Plugging this data into the formula yields:

Before: CDHP = (WEIGHT*MPH)/(823.3*CDTIME) = (3400*60)/(823.3*15) = 16.51 hp

After: CDHP = (3400*60)/(823.3*20) = 12.39 hp

Net Change: 16.51 - 12.39 = 4.12 hp @ 60 mph.

This formula indicates that the changes result in 4.12 hp less required to maintain the vehicle at 60 mph. Since aerodynamic drag varies with the square of speed, the effect will be greatly accentuated at higher speeds.

To minimize the effects of internal engine drag, coast down times for aerodynamics effects should be taken with the transmission in neutral.

When testing the effects of lubricants or the effects of accesory drag (an air conditiong compressor, for instance), leave the transmission engaged.

Coast down time should be measured on a flat, smooth, road with no wind or drafting, using a 2 way average, under similar atmospheric conditions.

I wouldn't put much faith in the absolute numbers provided by this formula, but I think it might be a good tool for assessing relative changes.