The satisfactory wiki says that the mk 1,2,3,4, & 5 belts are 1,2,4.5,8, & 13 items per second respectively.
The in game modded belts are 1.88, 3.75, 5.63, 9.38, & 13.13 items per second.

Is this to make an even number of items per tick, or was it unintentional?

Factorio belt speeds are a bit weird, but an internal value of 8 corresponds to 15 items per second. The slowest possible speed, therefore, is 1, which corresponds to 15/8 = 1.875 items per second. So that's what I had to use for Mk1 belts, and everything else is a multiple of that.

Are you sure? Wiki says speed is a double, and I thought I'd seen a powered-belt-mod once that used belts with a speed of 0 for stopping them when power was low. Kinda interested because if exactly 1/s isn't possible that ruins one of my own mod ideas :D.

Unfortunately yes, I'm sure. "Belt speed" is measured in number of tiles per tick, and belt positions are broken down into 256 positions per tile, and items must move an integer number of positions per tick.

This means the slowest possible belt speed is 1/256, which works out to 1.875 items/sec. Any value less than that is rounded down to 0. So yeah, 0 is possible, but it's usually not particularly useful!

Narf :(, thanks for clarifying (there goes my idea). Would you mind requesting that info be added to the wiki? The current explanation doesn't mention the even-position restriction.

https://wiki.factorio.com/Prototype/TransportBeltConnectable#speed

Done

Nice :)