Either way, they can withstand very high winds until the aerodynamic downforces are disrupted by flying debris (e.g., telephone poles, other vehicles or building materials) and the vehicle / occupants are hurled away. Using them for hail protection, against weak landspouts, as a last resort or for drama they are great, but not for actually trying to penetrate a violent tornado. I think it's important people realize this -- it's not safe to ride out a tornado in a vehicle. When Phil Henry built the first TIV in the mid-90's, we discussed this at great length, to avoid being over confident.