Yes, put surface goals anywhere and everywhere you want to know anything. It doesn't significantly increase solving time and lets you make the results plots you want.
I would recommend external flow for this simulation since the box walls will induce fluid friction if you put the car in a box and use internal flow. Then again, wind tunnels are basically boxes - I guess you just make the walls far enough away from the model that they don't have a significant effect on the results. I would then recommend you to try both and let us know what the difference is.
Remember to make use of symmetry if possible. Usually the left/right sides of a car are the same (pretty much)? This will cut calculation time by about 1/4.
Ok... But what kind of value should I expect?
The model i made made no much difference of downforce values at rear tyres with or without rear wing
Plus do i need to model a floor (to simulate road surface) ?
Any other criteria to make the simulation more reallistic? coz i know my model is quite too simple
i have yet to perfected my skills to draw a car model.
for reference i have attached a .ppt on what i did so far and results I obtained. I dont think result make sence.
The low Value of your Force might be, because you scaled your car down.
I cannot open your file, since i'm on 2009.
If so, you need to multiply the force by the scale-factor you used in your model.
the floor is important because you have a diffussor at the car end on the floor, the effect of the diffusor should be bigger than the big wing.
I would suggest you put in a simple solid body floor, to get the diffussor activated(->ground effects).
if you look from the side, the free air at the back is bigger - if the car moves forward, at this point it generates some underpressure wich results in a downforce.
your car has much more pressure at the front axle because the hood,and windshield acts like a huge downforcing wing.
without a rear wing your car could be lifted up at the rear axle.
.....thoughts of engineering