how can I make this more realistic

My suggestion: think what the story is. People like to tell themselves a story when they look at art.

Where did the people come from? Where did the car come from? You can show this part of the story with tracks. The missing tracks make the people look like statues and the car dropped by a helicopter - or added with Photoshop. If it is day, there should be shadows: the length of the shadows will tell the time of day.  The human figures are identical - why? Are they dancing?

From a composition point of view, the centre of a picture is key. People look there first. They find: a pool of darkness. Is that part of the story? If it's an accident, consider rearanging so the people OR the car (whichever you want to be the hero) is there. If they are equally important they could be opposite sides of the centre.

You have to ask how it got there?  Unless it was dropped by helicopter, it would have left some tire marks. We do imprints using Bevel & Emboss with Fill Opacity set to zero.  Plus some inner shadow.

Shadows are always crucial with composites.  We can see the brighter sky to the right, and we have some shadow/highlight clues on the dunes, so we try and replicate that with direction, softness and opacity.

BTW, I took your truck out, and put in a new one, because it is a zillion times easier to do this stuff with everything on its own layer.

Your truck was too light on what should be the shadow side.

Hi

A few suggestions:

1. Tone down the sky - it is unrealstically saturated - take a look at some sky photos *

2. Blur the sharp edges on the background dunes. Those in the foreground are sharp, the midground are softer therefore the background should be softer still

3. Look at the lighting on the car - it does not match the right/back lighting on the dunes. Same with the shadow.

* One other thing your photo was posted with no colour profile. When saving for web use - always export and Convert to sRGB and check the box to embed color profile. That way we can see the same colours you are seeing (assuming your monitor is correctly profiled).

Dave