Overall great renders, with a very nice mood to them. I'm not really good at rendering these kinds of things, but here are some noob suggestions anyway, which probably should be valued as such.
In both renders, you have a lot of noise going on. Try using BiDir rendering method if you're using PT now, and BiDir with MLT if you're already using BiDir. If the noise still won't go away, try raising the supersampling value to 3 or even 4 (it will eat more memory, but it shouldn't be that bad). Another thing that sometimes generates noise, is having too bright textures/materials. The wood for example, looks a bit bright. Check the wood texture in your photo editing software of choice and make sure that the RGB values don't go much higher than 200/200/200. Last but not least, use glass acceleration, if you aren't already.
In the night render, I'd suggest cranking up the HDRI a bit, so that there's a suggestion of the roofing and grass. I usually try to avoid having big areas totally black or totally white, but I suppose that's a matter of taste.
I'd try having a slightly more directional lighting situation in the daylight render. This can be achieved either by using the Indigo sun/sky model, or a HDRI with a brighter sun. The jeep looks maybe a tiny bit too small, but otherwise great.
Good luck, keep us updated!
