This is probably Luffy's best speed feat after the time skip.
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In the top right picture you see and explosion and with Luffy in the middle of it. Then We see Luffy appear behind CC and he has the following dialog. Waah that was close! That explosion was huge!! Then you see Luffy's expression and it looks like hes smiling. If Luffy actually had to try to dodge that, you would think his expression would be more serious or even tired. Hes not though.
Basically Luffy not even trying dodged an attack that fast with Gear 2nd. If you know anyone in physics you could try to calculate how fast that is. An explosion travels so fast and the size of it could be scales to normal size buildings and you could possibly calculate some speed out of it. Other then that you just have to use your imagination.