question about (geared) dc motors

[ballbreaker]

am trying to do a variation of a photovore without using any microchip or motor driver and to change speed it will have to change voltage from about 2,5V the lowest to 6V highest.

can a geared  dc motor operate in all this voltages  without loosing all the torque at 2,5V and not frying at 6V
and only really affect speed?

it doesn't matter if the geared motor is slow (but not to slow)

[maneuver]

I'm working on something simular, and asked a freind for help. Apparently yuo can run the solarbotic GM3 on a single 1.5 volt battery with enough torque

[ballbreaker]

nice thank you