By convention, internal vehicle friction refers to friction from a part of the kinematic chain: differential and hub friction, parasitic friction from brake pads when not braking. It does not include internal engine or gearbox friction which is integrated into the powertrain unit output.
The internal friction considered here is, as an initial approach, separate from speed (unlike friction from the powertrain which varies with the power to be supplied and the gear ratio, etc.)
For a mid-range passenger vehicle, this friction is equal to a constant force of around 50 N.
It is interesting to relate this value to 700 N, the rough sum of the resistant forces acting upon a vehicle of this type on flat ground at 90 km/h.
How to reduce internal friction
A significant reduction has already been achieved in the levels of friction in leaktight bearings.
The quality and temperature stability of lubricants has enabled advantageous gains to be made at differential level.
Moreover, automatic self-adjustment of play in brake pads has been optimized to enable a permanent reduction in friction.
Braking - a system with a high level of friction!
The friction intentionally caused by braking is obviously not part of internal friction.
We can simply point out the enormous gain already obtained with friction by using regenerative electric braking as is the case for stop and start systems and in electric vehicles.
For trucks, braking is still a major source of energy loss, including via use of the electric or hydraulic engine compression brakes that transform kinetic energy into heat.