100 troops with 0 crates will outrun 100 troops, 200 horses and 1 crate.
I think it's :
100 troops with 0 crates = speed 25 (less because number of troops influence speed)
100 troops, 200 horses and 1(to 199) crates = speed 38
As said Brutal "He just didn't specify 85% of what", let's
try to resume what i tested (not considering terrain) :
1 troop, 0 horse, 0 crate = speed 25
1 troop, 2 horses, 0 crate = speed 45 (180% of 25, or 130% with slow horse)
1 troop, 2 horses, 1 crate = speed 38 (85% of 45)
So for every horse (+1) you can carry 6 shields.
My fastest was a rouncey clocking at Speed: 99.100 meters per minute 1 guy 1 rouncey and no cargo!
With quick march?