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More math than I like to see on a Wed afternoon, but here is something to try out. You'll have to estimate the xc area, but that should be fun.
From the Bosch Automotive Handbook (4th ed):
Empirical determination of coefficients for aerodynamic drag and rolling resistance.
Allow vehicle to coast down in neutral under windless conditions on a level road surface. The time that elapses while the vehicle coasts down by a specific increment of speed is measured from two initial velocities, v1 (high speed) and v2 (low speed). This information is used to calculate the mean deceleration rates a1 and a2.
va1 - Initial velocity high speed (60 km/h used as example)
vb1 - Final velocity high speed (55 km/h)
t1 - High speed coast down time
va2 - Initial velocity low speed (15 km/h)
vb2 - Final velocity low speed (10 km/h)
t2 - Low speed coast down time
A - cross sectional area (meters squared)
m - mass of the car (kg)
Mean Velocity -
v1=(va1+vb1)/2
v2=(va2+vb2)/2
Mean Deceleration -
a1=(va1-vb1)/2
a2=(va2-vb2)/2
Drag Coeff -
cd=(6*m*(a1-a2))/(A*(v1^2-v2^2))
Coeff of rolling resistance -
f=(28.2*(a2*v1^2-a1*v2^2))/(1000*(v1^2-v2^2))
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