These are underexposed primarily because you are including a lot of bright sky in the composition and the camera is trying to bring that brightness down to a middle grey - thus pushing the tones of the car down into the mud.
Even if the day is overcast, try to shoot more towards the end so the light thru the clouds is not directly overhead.
Consciously look at the background and look at what shows through the windows of the car, then adjust your position accordingly.
Keep on shooting; it's the willingness to try that will make you good.