Can you tell how fast a hydraulic vane motor rotation speed, only by knowing the flow rate supplied to the motor?

No you need more, you need the motor displacement cui/rev, even pressure and viscosity, have an impact (on the volumetric efficiancy). There is a lot of Photo Non-Contact tachometer'shttp://www.reliabilitydirectstore.com/Tachometer-s/43.h...aF45YCFQ8QagodVGCKPg's out there to use if motor drive shaft is visible/accessable while running.

thanks mate,
do you happen to have the formula to do the calculation?

Pump speed (rpm) = Flow rate (gpm) x 231 / Displacement (cui per rev)

Divide the rpm's with the volumetric efficiency of your pump.....something like 95-98%....

The efficiency part of the story, pressure and vicosity etc try this sitePump efficiency math have fun with your spread sheet... :-)

thanks mate, you've been very helpfull.