To my knowledge, the fastest possible service speed is 234kmh when the service power is at 100%.
Don't know whether the extra height of John Isner gives some bonus to his service speed.
He served a 235kmh ace against me in the wimbledon final (difficulty level: incredible 1).
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Singles Titles and Finalists:
250: Kuala Lumpur '15
Challenger: Georgia F1 '14,Wroclaw '16
Doubles Titles and Finalists:
GS: Australian Open '15,Roland Garros '15(RU)
Masters: Paris '14(RU),Madrid '15(RU),Rome '15(RU)
250: Kuala Lumpur '14,Doha '16