gearing in scx 10 with 55 turn motor
 

What is best gear ratio for scx 10 spur and pinion gears with a 55 turn single motor on two cell lipo.

That will depend on what else your going to do to it. if you have the stock tire size then i would say 13 or 15. just keep an eye on the ESC and motor temps. if it's to hot to touch then your going to fry something. get a few pinions to keep on hand so if you notice it getting hot change it. if you get bigger tires then you'll have to change the pinion, spur or both to keep the temps down.

Is there a temp. range for brushed motors?? I run my brushless trucks at about 130-140 degrees or cooler if I can... just wondering.

anything over 160 is bad. heat will start to mess with the magnets in the motor. some motors can last longer that others when it comes to heat but why risk it. if you touch the motor and it's to hot to keep touching it then you need to change the gearing to lower temps.heat is the biggest enemy for anything electrical.