So what's your question? How I'm calculating the speed of the tire tread?

The speed of the tire tread that's in contact with the conveyor belt, relative to the plane that's bolted to the ground is equal to the speed of the conveyor belt, assuming that the tire isn't skidding.

The speed of the tire tread that's on the TOP of the tire is traveling at the negative speed of the conveyor belt.

So, if the conveyor is going 100mph towards the rear, the bottom of the tire tread is also going 100mph towards the rear, but the top of the tire tread is going 100mph towards the front.

But none of this matters anyway, because the wheel axle isn't moving. It's still bolted to the aircraft, which is bolted to the ground.