These are my findings from doing horse speed tests.

Wagon:

• Four 5-speed horses on a non-runed wagon will do approximately 26km/h when all four horses pass their soul checks for speed traits to kick in.
• Adding 88-90ql horseshoes (an average of 89.165ql; without Wind of Ages enchants) to the four horses resulted in 30.25km/h, or an increase of 4.25km/h.
• A 10% vehicle speed rune (rift wood, adamantine, Magranon) would be beneficial for a cart/wagon.