First of all, let me thank you for your help and your patience.
I've just understood what my mistake was. When you said "I've pushed a patch that allows you to set torque-split value", I somehow assumed that it was in the latest test build, and I installed it (I was previously using the 'stable' version that Sourceforge pushes by default). Now I see that it was here:
https://github.com/VDrift/vdrift/commit/...a5bdf6e4b7. I imagine that I should copy these changes into the files in my /physics folder, but I am a bit afraid of all those plus and minus signs... I think I'd rather wait for the next test build.
I'm very sorry for having wasted your time. You can now guess that I am not an IT guy.
By the way, the .car file for my project is almost ready... At the end, it will be a concept for the future Citroen C5. I already have solid data for engine, transmission, wheels and brakes; for the suspension I will certainly need the help of an expert tuner. But right now my immediate concern is to learn Blender from scratch. That's why I am not in a hurry in waiting for the next test build...