I've got 3.5" to the tow hook and 5" to the crossmember. It would be less but for bump strips and the curbs they always seem to have getting in and out of gas stations. I admire the young folks with their stanced this and air-bagged that.
The problem with highway speeds is ground strikes on bridges. Even a 1.5" discontinuity running across the traffic lanes will throw the wheels enough to bottom out.
Edit: last time I took the Wheatland Ferry the river was low, so I dragged pretty badly going on. The operator rammed the other bank at speed to force the ferry up the ramp to make it easier for me departure.
