I have been working on making removable loads for open hoppers and gondolas while waiting for glue or paint to dry on other projects. My layout has a gravel loading facility and a coal loading tipple, so lots of open hoppers and gondolas. Since gravel tends to weigh more than coal, roughly how much less should be in a car for a full load? Let’s presume we are using a 40’ offset-side car of 70 ton capacity. With coal the load will be piled up in a heap higher than the sides of the cars. Would a load of gravel be maybe 75% of that volume? 50%? I know there are differences in the weights depending upon size of the material and in the case of the stone what kind it is (limestone vs. granite, etc.) but what seems like a reasonable amount? Or does anyone else even bother with these differences when making loads?
The density of gravel (~100 lb/ft3) is about twice what it is for coal (35-60 lb/ft3 depending on the type) - so figure about half the load. The density of iron ore is even higher at 150-180 lb/ft3.
As you mention the material size has a lot of bearing on things. In my area there is a large limestone mine. I often see ore cars full to pretty much level to the top with 3-6” pieces of limestone rolling by.
that’s why @Gurkemeister provided densities. Something with twice the density will occupy half the space for the same weight.
also depends on the weight capacity of the hopper. hoppers designed for less dense coal are built less sturdy than hoppers designed for more dense materials
limestone has a higher density density has a higer density than gravel, 100-168 lb/ft3, so heavier limestone should occupy less volume than gravel tor the same weight, lighter limestone about the same volume
No, they’re not.
Cars are built to carry a certain tonnage. That’s it.
They may be built to a different size based on the density of the material they’re expected to carry. Giant hoppers for light wood chips, mid size hoppers for coal and other “standard” bulk materials. Tiny cars for heavy ore.
You can absolutely carry heavier materials in cars designed for coal, you just can’t fill them past the weight limit so they’ll be half-empty.
Most freight cars are all built to roughly the same weight capacity, with the size varying based on the density of what they’re intended to carry.
so wouldn’t a presumably less expensive hopper designed to carry less tonnage be used for coal?
Large limestone blocks would not pack as tightly as crushed granite or a similar fine material because there would be more voids in the load, regardless of the material’s density.
We had this exact problem on our HO layout. In searching for Accurail 70T 3-bay hoppers, we found some with UP stenciling that included a load line for rock loading. What with the different densities and the tapering load space, the stenciled line was about 1/3 of the carbodies height. We used a sander to trim the plastic Accurail loads so they would drop farther down into the load space and then detailed them to look more like crushed rock and less like coal.
We used colored strip decals to add load lines to the cars which did not have them.
I’ve noticed in photos of PRR lump iron ore loads in coal hoppers, they put the ore more towards the ends of the cars, more above the wheels. Apparently not to concentrate a dense load in the center which would likely be the weakest part of the car.
The inverse idea is behind ore cars: since iron ore is heavy relative to its size, ore cars are made very small so that they can be loaded to the max tonnage the trucks can handle with minimal wasted space.
It’s your layout so you can do whatever you prefer.
In my world it’s overthinking the situation. 
I repurposed some old hopper cars for my proto-free shortline railroad, to carry ballast.
I made a base with balsa wood, applied white glue, and added ballast, about halfway up the hopper.
No, you build a bigger car to carry more volume at the same tonnage. Why would you want to carry less tonnage?
less expensive, hopper made from aluminum, not smaller