So according to the Pathfinder Remaster,
Vehicles have size traits, but they don’t occupy the same spaces that most creatures use. Instead, each vehicle has specific dimensions provided in its stat block.
Most vehicles are Large or larger, and many vehicles are made for the purpose of carrying cargo. Unless stated otherwise, the amount of cargo a vehicle can carry depends on its size, terrain, and propulsion. A draft horse or similar creature can usually pull around 100 Bulk of goods consistently throughout the day, so pulled vehicles can typically hold 100 Bulk per Large creature pulling. Water vehicles, such as ships, have limits that are more based on volume than weight; a ship can hold upwards of 1,000 Bulk. Flying vehicles can typically hold only 1/10 the Bulk of a water vehicle and still remain airborne. The GM might rule that unique or unusual vehicles can hold different amounts of Bulk.
This presents a problem in that vehicles don't usually have large creatures pulling, pushing, rowing or otherwise moving them. This can still be used as a guideline. So for example, a Carriage is 10x10x7 and a Cart is 5x10x4, one is pulled by 1 Large creature the other by 2, so 100 and 200 bulk. These numbers are applied to either (2x2x1.5)=6 five foot cubes or (1x2x.8)=1.6 five foot cubes, though you can also just take the dimensions and divide by 125. Given that, values of 30 to 50 Bulk per 5 foot cube.
What actually is Bulk?
The book says 10 lbs but it also says a medium creature is 6 bulk, which suggests more like 20 to 30 lbs (20 would make medium creatures 120 lbs, and 30 would make them 180). It also says a large creature is 12 Bulk. (which is anywhere from the canon 120 to my recalulated 360). To be clear, bulk should be a range based on a nebulous combination of length and weight, not just one particular weight or one particular length.
My initial "gut reaction" of 25 bulk per 5 foot square is reasonable when taken in consideration for the fact that vehicles' own Bulk isn't considered for its weight limit, but I'm inclined to consider the following
- (50xCubes)-(Bulk of Creature listed in Player Core p. 269) Bulk for the same size creature (500 to 1500 lbs per cube)
- 25 to 30 bulk per cube ignoring the "Bulk of a Creature" appropriate to a vehicle of the creature size (250-900 lbs per cube)
- Water vehicles basing their cargo on volume rather than weight still hold 10x the volume of land vehicles calculated this way (e.g. one of the above calculations).
