dont know exactly what you're after but i've had no trouble keeping mine moving using the standard 3.7 open centre and 185/60/13's and 195/50/15's .... now with a new 4.11 lsd and 185/45/15's it's still goes just as well ....
the A12 isn't a monster for torque so it's got to be driven higher in the revs to keep the momentum up and not rely on having the motor push the car up to speed from low revs.
what have you done to your motor ?? cam, head, bottom end etc ??