The Top Speeds of the karts depend on their category (Lightweight, Middleweight, or Heavyweight). Internally, they are computed in m/s, but we will speak about them in km/h. You can also find in the resources or in my Stk fork mods to have a numeric speedometer.
Here are the Top Speeds in SuperTux:
Lightweights: 85.5 km/h Middleweight: 90.0 km/h Heavyweight: 94.5 km/h
You can see why Heavyweights perform better in Speedruns in general. Their poor acceleration is largely compensated by their massive 5% speed advantage over Middleweights.
The Skid Bonus invariably adds 16.2 km/h for the yellow, and 23.4 km/h for the red.
The Nitro adds:
Lightweights: +16.2 km/h Middleweight: +18.0 km/h Heavyweight: +19.8 km/h
However, one nitro press consumes more Nitro for Heavyweights (and less for Lightweights)
These bonuses don't increase instantly your speed, but rather increase it gradually until it reaches for example +19.8 km/h. Nitro and Skidding stack up so if both are used and at their maximum, your speed is boosted by 23.4 (red) + 19.8 = +43.2 km/h. So Pidgin with Red Skid Bonus and Nitro has a top speed of 137.7 km/h. For Kiki, it would only be 117.9 km/h.
Zipper mechanic is harder to study, this will be completed later.
========
The Boost mechanics are pretty weird in general. This guide will present 2 exploits of it.
- Getting the Skidding Bonus during a Zipper Boost instantly increases your speed.
This trick is what allowed the first Sub 1' in Oliver's Math Class.
If you release skidding and get the bonus just after using a Zipper, your (max) speed is instantly increased by about 8 km/h. That means that it is be better to release the skidding the earliest possible during the Zipper boost (or alternatively, delay smartly Zipper uses), to profit from a higher max speed longer. And if you are quick enough, you can repeat this with another Skid Bonus to increase further the max speed. You really get a better boost, and not just a staggered bonus.
- Using Nitro when Zipper or Skid Boost is wearing off is beneficial.
If you use Nitro alone, the speed increases slowly until a certain point, like said before. But if you use it during a Zipper wear off, the speed increase will not start from the "beginning", but from your current speed.
Concretely, let's say we are using Pidgin. If we use nitro while not having any other bonus in effect (at 94.5 km/h), the speed will increase very slowly from 94.5 km/h to 114.3 km/h. Now, if we have a Zipper wearing off, let's say we are now at 110 km/h and the speed is decreasing. We use Nitro: the speed will stop decreasing and increase starting from (a bit less than) 110 km/h, so we reach the top speed 114.3 km/h much faster and profit from it much longer (without Zipper, we are at 114.3 km/h only during a very short instant). If the effects were simply stacking up, the speed should decrease from 110 km/h (the wearing off being much faster than the nitro speed increase), reach a bit more than 94.5 km/h (due to the Nitro boost effect that just started), and then increase back to 114.3 km/h. But it is not what happens for some reason.
That also applies for using Nitro when a Skid Bonus is wearing off and probably other Boost combinations. When Zipper or Skid Bonus are in contrary increasing the speed, it looks like that using Nitro just stacks up the boost without side effects.
