I don’t think I did anything too weird with this car, BUT I couldn’t get a bunch of fixtures to show up (bumpers, and that black rectangle under misc). Would they really cause this sort of problem though? I’ll take another look tomorrow but it’s getting a bit late here, see if I can figure out what about this car is causing problems. I don’t think it’s my computer, because I threw together another car and that calculated correctly.
Those Fixtures are missing something in UE4 called “CPU Access”.
It’s a known issue, and will be fixed in a hotfix.