How hard the hit must be to destroy the object, just write the number. Imagine a minimum collision force where the object is destroyed, and calculate the threshold with this:
[Speed x Mass x 0.025 = Threshold]

Example 1: Object will be destroyed when a 1.5-ton-vehicle hits it at 100 mph. What is the threshold? Speed = 100 (mph, or whatever the Carmageddon velocity units are) Mass = 1.5 (tons) Threshold = ? Speed x Mass x 0.025 = Threshold -> 100 x 1.5 x 0.025 = 3.75 Answer: The threshold is 3.75.
Example 2: How fast must a 5-ton vehicle drive, to destroy a object which threshold is 1000? Threshold = 1000 Mass = 5 (tons) Speed = ? (mph) Speed = Threshold (0.025 * Mass) -> Speed = 1000 (0.025 * 5) = 1000 0.125 = 8000 (mph)
Finally, all three variations: 1. Threshold = Speed x Mass x 0.025 2. Speed = Threshold (Mass x 0.025) 3. Mass = Threshold (Speed x 0.025)