Mana Games

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).

He is higher than the maximum original limit considered by the game, so that gives him a service speed boost.

The Max service speed I've made with my player was 238 km/h but he has a height of 213 cm.
But sometimes I have bugs where I serve up to 2298 km/h.