If the vehicle is standing still it can take as much as 100 MPH or more in wind speed to flip over a car completely.
A tornado can certainly flip over a car very easily and even pick it up and throw it through the air.
Of course if you're driving down the road then even 70 MPH or high winds could eventually push your car over but most times the wind if high enough will blow your car into an oncoming lane or cause you to lose control of the vehicle and blow you off the road.
Cars are pretty heavy so it will take a good wind force to flip over cars if they're not going down the road but it's very possible for 100 to 150 MPH winds to blow over a car.