K, Here's the dice:
If you're not using layers, start noe, because otherwise you're going to have the worst time everytime you try to do something to something small.
The thing to do (Good selecting by the way!) is to use the polygonal lasso which you have been, select the car's body (Exclude the wheels!), like you have been, now copy this selection onto a new layer (Ctrl+C, then create a new layer, then Paste onto the new layer, I think you may just be able to use Ctrl+C, and Ctrl+V and it'll make a new layer for you. Depends on your version of PS)
Once you have the new layer with the car on it, start moving it down to where you want it, you should see the same image you posted above after this step.
Now is where your artistic skills are required...
See the stamp tool on your toolbar? (Looks like a stamp with a triangle in the middle of it.) It's called the cloning stamp. Select it, then go to your first layer (The one with the background on it). Now, adjust the size of the brush in relation to the field you're working in... IN this case, I am choosing a brush that is about the width of the path on the left, I may change brush size though.
Now, press and hold the Alt key down, and click somewhere with a pattern on the image that you want to paint onto another piece of the image. In this case I am starting from left to right, try to make it look as realistic as possible, as here is where it is that you can lose the realism easily...
(TIP: Use single clicks instead of strokes for the cloning stamp.)
Now I have continued on to where I now have the path going straight into the cars roof.
(TIP: Use a solid brush for solid things that don't have blurred edges, if necessary, use the blur tool later to fix it.)
Okay, now I have almost finished the cloning, all that is left is to rebuild the other side of the path, and fix imperfections.
Notice in the red circles the two imperfections? On the left circle, the colors don't flow, they're what I call "Jagged colors". In the right side, the grass is too cloned, it's the same, it's not random. Now I will address these issues:
Fixing Jagged Colors:
When you end up with Jagged colors, the easiest and one of the most effective ways to correct this issue is to simply use the smudge tool. Select the smudge tool (Click and hold on the blur tool and select the smudge tool), and pick a fairly small brush with a gradual (blurred) edge. Now in small accurate strokes, pull one color onto the other, repeat tis until you have finished the whole color. Your ultimate goal? To make those colors flow into each other.
Producing a more random cloning pattern:
When you need to fix a part of an image that is covered witha noticeably same pattern, simply select the clone tool, go to different parts of the image with the pattern that you're looking for, and go over some of the spots that are the same. Experiment, and take your time, you can produce an almost real background for your car to sit on.
I have now corrected this problem, and here is the car's current state:
What's left? Well the top of the car is too sharp for the background image in terms of quality of the JPEG, so let's blur it a little bit to get it to match a bit better.
Select the blur tool, use a small brush, I am using a brush with a diameter of 5 pixels, and a gradual (blurred) edge. Now, switch to the layer with the car on it. Start blurring the edges of the car to match the bottom's blur. Take it slow, and try not to blur the car's paint, but instead just the edge.
Here is what I have come up with, my final product.
Let me know if there's anything I can do for you to change it a bit, or explain something in a bit more detail.
Have fun, and good look Photoshopping! You certainly have potential, as you have meticulously selected the edges of this car, which a lot of people don't. Selecting can make or break an image, and you certainly have the right way of doing it down pat!
Again, Have fun! Hope this helps a bit.
