What is the slowest speed at which a vehicle driven off a cliff can clear the cliff without catching its underside between the wheels?

In the usual frictionless vacuum, this would appear to depend on the vehicle's clearance and its wheelbase. The front axle responds to gravitational attraction as soon as it clears the cliff, and the vehicle rotates on its rear axle. For a vehicle of 12-foot wheelbase and 1 foot clearance, I've calculated about 12.8mph (wheels are of infinitesimal diameter and mass is distributed evenly from the front to the back of the vehicle, which has 0 overhang).

I've been unable to develop a formula yielding speed for wheelbase and clearance, though using Excel's Goal Seek function and the derivative of the function I have developed (which includes t), I'm able to iterate toward higher levels of precision.

Any thoughts, ideas, experience?