First off I am warning you now... DO NOT USE THE STOCK ELECTRONICS!
Sometimes the HL tanks malfunction and lose control and the tank stops still and just starts rapid firing the airsoft gun...and you have to run and go shut the tank off ..... If this happens with a CO2 real BB gun it will be VERY dangerous!!
Especially if there are any people around where you are driving your tank!!

Second on the servo..if you use real electronics and a real RC radio you can controle the speed of the servo because it's digital proportional.