all of the squares should be pretty much the same size , but for example If I wanted more details on the hood, i could increase the size of the hood uv island within the uv layout, but generally with cars and such i keep the size of the uv islands equal, so the texel density is even over the model
"....model will be around 60,000 tris, but will have no undercarriage" It's mesh is basically a rectangle so a low tri count including interiors would imo be expected, although just a slight nitpick thought for future reference. On your ChevelIe for example, I do think that spending a few more polys especially around the…