As Ken notes, proper toe-in is a key consideration.
When I first got my Amphi, driving at speed was downright terrifying.* I recall thinking that if this is the way an Amphicar handles, I am going to get rid of it before I put anyone's life at risk.* I switched to radial tires but that did not help much.* Tried a bunch of other things, it was still bad.* Finally, checked the toe-in using a ruler and a string (thanks Billy Syx).* Once the toe was set correctly she suddenly drove like a normal car.
When I first got my Amphi, driving at speed was downright terrifying.* I recall thinking that if this is the way an Amphicar handles, I am going to get rid of it before I put anyone's life at risk.* I switched to radial tires but that did not help much.* Tried a bunch of other things, it was still bad.* Finally, checked the toe-in using a ruler and a string (thanks Billy Syx).* Once the toe was set correctly she suddenly drove like a normal car.