When I zoom out pieces of my model disappear.

This problem has been bugging me a lot because whenever I zoom out to get a good look at what I am doing my wheels would disappear along with some other parts.

This is the jeep zoomed out.

This is when it is zoomed in a little more

This is zoomed in where I can see the back wheels now, but the rest of the car is now not in my view. 

If anybody has an answer, please respond I hate having this problem.

Edu