Credit to SSJRyu1.
Based on SSJRyu1's calc method: I used the same basis and used the radius of the new universe:
- Blog: 1.76x10^27m
(basically the same text)
Goku and Pikkon Cross the Afterlife[]
This is one amazing speed feat that base Goku and Pikkon are able to do. This is before the 7 year time skip between the Cell games and the Buu saga. They fly from Grand Kai's Planet, down to Hell.
Distance[]
Now why is this amazing? Because it is established that the afterlife is easily at least the size of the Universe.
And Grand Kai's planet is above heaven in every depiction and map, meaning they crossed down to heaven, across heaven, and down to Hell.
To conservatively low-ball, we will use 1/2 the diameter of the New Universe, 1.76x10^27m/2.
Time frame[]
As for time frame, no specific time frame is given, but it was a short time as Cell and Frieza were an immediate threat that they arrived to take care of before it got out of control, and came back from the battle in a relatively short time frame as well. We will low ball and use 10 min flight time to reach Hell.
Speed[]
So: (distance traveled)/(estimated time frame) = ((1.76x10^27m)/2)/ 600s = 1.46e24m/s
Speed = 1.46e24m/s = 4.38 Quadrillion c
Would be higher with transformations and other increases.