1. Tire growth due to centrifugal force at high speeds, although the steel radial construction should be more stable.
2. Suspension droop (downward movement) since the shackles are at the outer ends of the springs, right? The inner ends of the spring are fixed at the equalizer, so as the springs shorten under droop, the axles will tend to move closer to each other.
I'm not saying the tires WILL touch - just that you have less than the 1"-2" clearance between the tires that is normally desirable.
Rusty