they dont do v max,

(going off what g force do) usually gently wind it up in each gear into 4th (4th is usually 1:1 ratio and they can cal the transmission losses best that way and the torque, then hoof it in 4th from fairly low down and see how it comes on boost (fuelling and boost are monitored - if either go towards dangerous they stop the run) and continue until its near the to of the revs - so no different than a good 4th gear motorway overtake manouvoure - pulling 110ish (allegedly occifer )

the whole run (once strapped down) takes about 1 minute - the operator should check all the tyre pressures are coreect and equal before the run - it up to you to make sure oil/water levels etc are 100% before hand.

hth