FPS problem when increasing the no of vehicles

hi,
when iam increasing the no. of vehciles from 7 to 12 my fps goes to 60 to 20,if i comment setmassmatrix method my fps will be good but vehicle usercallback will not call so vehicle will not move by applying the torque,please tell me how can i optimize my fps fall in my project,one more thing my fps decreases because of physics not for models.
thx in advanc

How often do you call setmassmatrix?

hi thx for the reply i call only once.but if i comment setmassmatrix my usercallback of simplevehicle will not call,is there any solution to decrease the fps fall for adding the more vehicles