I can't check right now, but can you make the gravity positive to lift the car as high as you'd like, then change it back to watch it fall for miles?
You can make it negative (A positve value), but you have to reset the car for changes in gravity to take effect.
i noticed that around 6000km/h the cars start to destroy themselves, the pickup can go up to 7000 without going so destroyed, but the ibusu supricingly the roof gives up first hmm... i would had suspected that 1000km/h would been enough to let the air literally rip the car aparts... Never stops supricing heh