[MrForbes]

It's pretty hard to come up with an equation that will tell you what force it will take to overcome rolling resistance and aerodynamic drag...generally these numbers are found empirically, and then an equation is made up with the necessary constants.

Here are some equations I swiped from the internets, it's a NASA page about hybrids so I think it's public domain...
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A-1. Aerodynamic Drag Losses

The following equation calculates power loss due to aerodynamic drag, represented by the variable Paero(Watts).

Eq. for aerodynamic drag P_drag = (A_frontal x C_drag x (Density) x V^2)/2

Afrontal = Vehicle frontal area (m2)

CD = coefficient of drag of vehicle

V = vehicle velocity (m/s)

rhoair = atmospheric density, kg/m3


The following equation calculates loss required to overcome rolling resistance, represented by Proll.

Proll = MGr.Veh g (R0 + R1 V + R2 V2 + R3V3) V

MGr.Veh. = gross vehicle mass (kg).

g = acceleration due to gravity (m/s2)

R0, 1, 2 = rolling resistance coefficients