The basic idea is to give vehicles somewhat realistic stats, and not just put 100 free slots on all of them.

Now, there are mods like that for older versions of the game, but they do not work (at least for me) for more recent patches, and has not been updated for a long time.

So the mod tries to give all the vehicles more realistic storage space as well as the durability stats.
For example: obviously, that the cargo van must have more space than a sports car, which has no trunk at all, and can only fit stuff into the back seat.
Having that in mind, as a measuring unit, a standard 50 liter camping backpack was taken, which itself can hold about two or three AK-47's, several water bottles, some clothing and boots, and minor junk. The average van cargo area is about: 1,4m wide, 1,6m in height and 2,6m in depth. So reasonably, we can throw there about 16 of our backpacks.
Now, the only problem is, that in the game, a single handgun takes exactly the same amount space as our backpack mentioned above, so we try to account for that too. So when converting those filled backpacks into the 'game slots' our biggest vehicle will have 90 free slots, and the smallest one only 9.
Obviously this will not be super realistic mod, but in my opinion, its close enough.

Also, all vehicles get a little durability boost. This does not mean, that you can plow thru five hordes of zombies in a single drive-by, but two or three will not make your car instantly explode.

How to install:
Copy the .pak file into the "Paks" directory. Steam version example below.
C:\Users\username\AppData\Local\StateOfDecay2\Saved\Paks\

Will conflict/not work with other mods that edit vehicle inventory/durability settings.