I noticed that the red belts are a little faster than expected, and so ran some tests, and it seems they are exactly 3 (instead of 2) times faster the yellow belts. Not sure if this is a bug with the mod or due to rounding errors on the Factorio end?

I tested this by building a 10-length yellow belt, then a 30-length red belt, dropping an item at the start, and observing that they reach the end at the same time.

I also noticed that the items slip a little bit on the yellow belts (but less so on the red ones).

Ok, I did some testing, and it seems that Factorio is rounding the belt speed down to multiples of 0.125, which explains why 0.2 speed doesn't work as expected. Here's what I tested:

• At 0.2 speed, we have the yellow belts running ~1/3 the speed of red belts.
• Same with 0.20001
• Same at 0.21
• At 0.25 both belts now work fine!
• At 0.1 the yellow belts no longer work at all (items on them do not move), red belts are like yellow were before.
• At 0.125 both belts work fine.
• At 0.25 both belts work fine.

So, currently the claimed speeds are 0.2 and 0.4 (of a standard yellow belt), but instead we are getting 0.125, and 0.375, which is why red is 3x faster, and also explains the sliding. In my version, I've updated the belt modifier to 0.125, which keeps the yellow belts at the same speed but slows the red belts down to 0.25.

Thats explains it! A few of us tried to get to the bottom of it, but we never clicked to it being multiples of 12.5% .