i would assume its the maximum torque rating of the engine or electric motor alone

Thanks a lot for your answer.
I got this website in the past, but there's no clarification about the type of motor torque to consider in our calculation.
The a.m. website reports:
"The torque on the driving axle creates a force between the tyres and the road which is used to propel the vehicle."
Which torque? Should I use the stall torque (==> locked rotor torque) or the "max traction torque S2 5min at 2km/h" or should it be referred to a certain acceleration?
Thanks again.

do a google search for "drawbar pull"
use the quotation marks
the 1st link (from webtec DOT co DOT uk) gives the calc in imp and metric