The Michelin's are R1W tread which is deeper than the Firestone's. Logic would seem that a deeper lug would be an advantage but in reality a short lug in firm soil is actually more efficient. On hard ground, a half worn out tire will pull more than a full tread. Mud or soft soil is a different story, but firm soil with proper weighting, short bars are better. Tractor pullers don't use big deep tread.... they are on a firm track. The Michelin's with deeper tread were at a disadvantage.

We buy tires with deep tread for good reasons, but pulling the most efficiently on hard or firm soil is not one of them.

John