You can find an average of the negative forces by decelerating. Take your car up to say 100mph, then at a set point let off the gas and let it cost to a stop. Time it, and take the distance it took you to stop. That will give you the average force needed to overcome to get to 100 mph approx.
F=MA
Mass= your car
Acceleration will be the Distance/Time
This force will include your rolling friction, air friction etc.
It will not incluse power train loss.