My current servos are as follows.
Spektrum Z270 for throttle
Torque: 43 oz/in @ 4.8V, 49 oz/in @ 6V
Speed: .23 sec/60° @ 4.8V, .19 sec/60° @ 6V
Dimensions (WxLxH): 0.73" x 1.55" x 1.46"
Weight: 1.50 oz
Motor Type: 3-pole ferrite
Gears: Nylon
Spektrum Z590 for steering
Torque 85 oz/in at 6V
Speed .15 sec/60° at 6V
Dimensions 0.73 x 1.55 x 1.46in
Weight 1.6 oz
Obviously any of the following would be more than enough as the 7940 has almost double the torque and is twice as fast as the Z590 that is currently on it.
I'm not real sure of the difference of these servos other than the obvious speed and torque.
Other than the ~5 dollars why wouldn't I use a 7950TG over the 7955TG? The 7950TG is quicker than has more torque.
http://www.servocity.com/html/hs-7940tg_servo.html
http://www.servocity.com/html/hs-7950tg_servo.html
http://www.servocity.com/html/hs-7955tg_servo.html
Also wouldn't I want to use the 7940 for the throttle and the brake and the 7950 or the 7955 for the steering?