Power Balance
The traction force FX is defined as the driving force acting at the tire treads of the driven wheels. A relation for the traction force can be determined from the driving torque available at the wheel hubs, also called the hub torque MH, as follows:

Eq. 3-1

Considering the torque loss Mloss arising in the drive train from the engine to the wheel hub as well as due to the transmission and differential ratios, the hub torque is determined from the engine torque Meng as follows:

Eq. 3-2

The difference Meng - Mloss is called net torque Mn. The net torque is only relevant from the point of view of calculations and converts the hub torque to the engine speed.
Speed, torque, transmission ratio and oil temperature have an essential influence on the torque loss. As an initial point, the torque loss can be assumed to be constant for a specific gear ratio over speed (see Fig. 3-1).
In order to estimate the magnitude of the constant torque loss, a drive train nominal efficiency η0 is defined. This nominal efficiency describes the torque loss as a proportion of the engine torque at nominal power.
constant for one gear level Eq. 3-3

(engine torque at nominal power) Eq. 3-4

The figure shows the transmission efficiencies of a passenger car manual transmission measured in the third gear for different input torques. They can be approximated by straight lines.
In the presented example, at an efficiency of approx. 95%, a constant torque loss of (1-0.95)·350 Nm=17.5 Nm for a nominal torque of 350 Nm results.