Difference between Force and Normal Force goals

Can anyone explain the difference between force and normal force global goals?

I'm trying to find the drag force of a car I'm simulating... there is a discrepancy of about 20% between the different goals, I'm not sure which I should use!

Also is there any way to measure downforce seperately at the front and rear? I can get an overal value with Y force, but no front/rear...

Many Thanks,

James

SolidworksFlow Simulation