The wiki lists vehicle speeds in units of pixels per day. This article details how they are computed.
Each land vehicle or ship has a base speed and a weight effect, which are listed on the vehicle's own wiki page.
The base speed determines the speed at which the vehicle moves, were there no load. A vehicle with a larger base speed will usually go faster than a vehicle with a smaller base speed. Base speeds of ships and land vehicles are not comparable, because the land vehicle's base speed must be multiplied by the road factor.
The weight effect determines how sensitive a vehicle's speed is to the load on the vehicle. A vehicle with a larger weight effect will be slowed less by a heavy load.
The load on a vehicle is determined by adding the weights of all the items on board. Objects and raw materials are added wherever they are. Machinery, including engines and locks, are also counted in the load. Passengers are counted as weighing 60000 grams each.
Land vehicle speeds (but not walking) are affected by the type of road being traveled on. A list of road types, and their associated road factors: Types of roads
A character with an empty inventory travels at a speed of 10 units per day on any road type. Tiredness and damage have no effect on traveling speed (this is true for vehicles as well).
Walking speed = 10 - (inventory weight)/5000
Vehicle speed = (base speed)*(road factor) - (load / weight effect)
Ship speed = (base speed) - (load / weight effect)