I run in the 115 MPH range with radials up front ( P235/60's ). If I try and pump them up ( over 35 psi ) the car gets squirrely. Seems to be best around 32psi. Run bias ply Hoosiers out back. I don't think there is any "connection" between poor handling at speed and running radial / bias tires as long as they are on different axles. I do think bias ply tires up front add stability at speed and radials are more sensitive to tire pressure than bias tires. When you are running a big old boat at speed you gotta make sure all the suspension and steering components are up to speed - so to speak.