Where does it top out?

Somewhere between 140 and 250 MPH. GSX tops a 187 GST tops at 139, but you can always take out the govenor and top at 187 like the gsx GS-T tops at 135 because factory tires are rated for 135 only. GSX has no governor because factory tires were good for 160, and the car only has enough power to get it to about 145 downhill. besides, GSX redlines 5th at 156 anyway.