Car  Required Power and Torque
Power, torque, efficiency and wheel force acting on a car.
Engine Power
Required power from an engine to keep a car at constant speed can be calculated as
P = F_{T} v / η (1)
where
P = engine power (W)
F_{T} = total forces acting on the car  rolling resistance force, gradient resistance force and aerodynamic drag resistance (N)
v = velocity of the car (m/s)
η = overall efficiency in the transmission, normally ranging 0.85 (low gear)  0.9 (direct drive)
For a car that accelerates the acceleration force must be added to the total force.
Example  Car and required Engine Power
The required engine power for a car driving on a flat surface with constant speed 90 km/h with an aerodynamic resistance force 250 N and rolling resistance force 400 N and overall efficiency 0.85  can be calculated as
P = ((250 N) + (400 N)) (90 km/h) (1000 m/km) (1/3600 h/s) / 0.85
= 19118 W
= 19 kW
Engine Torque or Moment
Motor torque vs. power and rpm can be calculated
T = P / (2 π n_{rps})
= 0.159 P / n_{rps}
= P / (2 π (n_{rpm }/ 60))
= 9.55 P / n_{rpm } (2)
where
T = torque or moment (Nm)
n_{rps} = engine speed (rps, rev/sec)
n_{rpm} = engine speed (rpm, rev/min)
Example  Car and required Engine Moment
The moment delivered by the motor in the car above with the engine running at speed 1500 rpm can be calculated as
T = 9.55 (19118 W) / (1500 rpm)
= 121 Nm
Wheel Force
The total force (1) acting on the car is equal to the traction force between the driving wheels and the road surface:
F_{w} = F_{T }
where
F_{w} = force acting between driving wheels and road surface (N)
The traction force can be expressed with engine torque and velocity and wheels sizes and velocities:
F_{w} = F_{T }
= (T η / r) (n_{rps} / n_{w_rps})
= (T η / r) (n_{rpm} / n_{w_rpm})
= (2 T η / d) (n_{rpm} / n_{w_rpm}) _{ }(3)
r = wheel radius (m)
d = wheel diameter (m)
n_{w_rps }= wheel speed (rps, rev/sec)
n_{w_rpm }= wheel speed (rpm, rev/min)
Note that curved driving adds a centripetal force to the total force acting between the wheels and the road surface.
For power required for inclination  check car example at the end of "Forces Acting on Body Moving on an Inclined Plane".
Related Topics

Dynamics
Motion  velocity and acceleration, forces and torque.
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