This is the 5189th most frequent Polish word.
prędkością
"Prędkością" means "with (a) speed" or "by (a) speed" in English.
The word 'prędkością' is in the instrumental case, indicating the manner or means by which the action occurred (with high speed).
Samochód poruszał się z dużą prędkością po autostradzie.
The car was moving at high speed on the highway.
The word 'prędkości' appears in the genitive plural (or singular genitive depending on context), referring to measures or degrees of speed.
Napęd nowoczesnych maszyn opiera się na różnicach prędkości.
The drive of modern machines is based on speed differences.
Here, 'prędkością' is in the instrumental case again, illustrating the property or characteristic being described.