I saw on the description that the last two tires of sorters have fixed speeds, can you adjust the speeds to those of the belts? bobs mod allows you to adjust it, mine is 18.75 per tier

Hi there,
The belt are easily configurable, but the beltSorter are highly complex scripted entities. To be efficient they only run a script after certain amount of update ticks. I would be very hard to make this easily configurable. Thus I probably won't find the time to implement this. Feel free to try around with the scripts: control/belt-sorter.lua -> local maxUpdateTicks = {32,16,11,8,6}
Factorio has 60 ticks per second. This array specifies the ticks until beltSorter moves all items again from source to target.

OK, thanks

Anyway, if by any chance you do find a moment for this, I leave you here an explanation of how Bob handled those issues for his belts

Items per second is a complex calculations involving pixels per second at standard zoom, and the fact that a belt holds 4 items per tile in each of the 2 lanes.
15 turns out to be a perfectly round number when you consider 8 items per tile, 60 ticks per second and 32 pixels per tile.
divide that 15 by 8 and you get 1.875. this means that the belt is moving 1/8th of a pixel per tick.
now consider 256 is a computer number, divide that by 32 (pixels per tile) and you get... 8.
so 1.875 is 1/256th of a tile.

Considering all this, it's fairly obvious that internally, objects on a belt are moving in increments of 8 256th of a tile at speed 15, which is why increments of only 1.875 IPS work correctly.