How are you tieing down your car in your enclosed trailer

Ddaddy said

That assumes a rigid (no stretch) attachment like a chain or a bar. Straps are not rigid and have stretch. Therefore, they spread the load as the strap stretches with acute load changes and return to original tension if they have appropriate elasticity.

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The force is constant from one end to the other and does not 'spread' or change versus length. Nor does the strain change IF the same force is achieved on the strap. If you think that you are getting less force on the longer strap, it is not due to any 'spreading' but due to putting less force on it period!

Stretch appears to be being confused with force and strain here. The stretch is the accumulated length changes due to the strain. If the same force and thus strain is achieved, then the stretch will be longer on the longer strap. Conversely, if the longer strap is stretched the same physical amount as a shorter strap, that is ONLY because a lower force and thus a lower strain is being applied.

BTW, the same applies for straps and bars and chains; they all have strain and stretch.