Let's take a simple car.

I prefer to start with repeating components, as it is usually the most effective way.
If your model is a 1 single component, you would have to explode it, to make several components
Let's put all wheels on 1 layer, so we can see only wheels and nothing more.
It's not important, but it helps me when I work with large models, you may delete the created layers when work done.
I wouldn't explain how to work with layers, do some research, just show you a picture.

Now explode all wheels so they are not in group or component, just a separate parts.
Completely select one of the wheels and press G on your keyboard. "Create Component" window should pop up. Name it "Wheel" and click "Create". Then select other wheel and do the same. A confirmation window should pop up
"A component with that name already exists. Do you want to replace that component?"
Click "Yes"
Do the same with other 2 wheels, make sure you name them with the same name.
Now you should get all wheels facing to one side. That is not a big problem, we can fix it in just a few clicks. Click right button of your mouth on the wheel that is facing wrong direction, select "Flip Along" and then select red, green or blue axes you want to flip it by. In our case it is red.
And last important step purge unused components.
Click Window on the SketchUp main menu, select "Model Info", than select "Statistics" and click "Purge Unused". Now you can close the window, and save the model.
It may drop the file size to a few kilos, depends how heavy the wheels were.
Also you could try the same technique with front and rear lights, mirrors and doors if they are separate components.
Part 2 is more difficult and less effective. It is based on cutting in half symmetrical components.