How to estimate the frictional losses of a car drivetrain?

I’m making a car driving video game which uses real engine torque curves, gear ratios and other physical attributes as parameters to simulate the motion of a specific car.

The problem here is that I don’t quite understand the way the frictional torque of transmission components (gearbox, drive shafts etc.) builds up as the car accelerates through gears. I’m not trying to calculate the torque caused by individual components in the drivetrain but instead setting an imaginary parameter that would reflect the overall friction properties of a specific transmission eg. setting a higher value for a heavy-duty vehicle transmission than for a small passenger car.

What I’m seeking here is a simplified function to estimate the torque applied to drive wheels caused by drivetrain power losses. I have so far tried using a constant value for the above-mentioned torque. That gives somewhat realistic acceleration having wide open throttle and running the engine at maximum power range (typically shifting up near engine redline). However, the simulated car struggles to accelerate at a lower “casual driving” rpm range (2000-3500 rpm for a petrol engine) and slows down at a relatively gradual uphill road when compared to the real life counterpart of the simulated car.