Hay Kenmo,
As Michael said, MoI is not very good solution to model cars inside it, because of that "Organic" type of objects.
I used to work a lot in MoI but because of that limitation we created Rocket 3F, very simple tool for that kind of organic objects.
So if you love to work in MoI you can use Rocket 3F to create organic objects inside it than send it to MoI in one click. Rocket has bridge for this.
Check this video, (now instaling Bridge it is much more simpler than on video):
https://youtu.be/ZzdAmzCaDZU

I also created some car like yours inside Rocket: