I have some problems like this one in my game sometimes, with newton engine . But basically always the alignment, grouping, absurd physical properties , and sometimes the file saving issue I comment further, that make it happen.

With and without sky, I had this problem. Basically, I compiled 500 times to have a good result trying to figure out how it works. When u say "my origin is in the air" are you talking that you are placing the car close to the ground or touching it ? Or, that newton only works when you let the car fall in the ground ?You must must let the car fall a bit.

When you change your grouping etc etc ,Save the level, close wed, sometimes make even a winxp logoff, open wed again, and try to compile/run the level. I figured that sometimes if you dont restart WED the Newton engine will keep in error even when u did everything correctly.

Hope this can help, I´m new to all this.