If you have a Mamba Max speedo already then just buy the Orion 10.5 motor and your ready. I ran last night with the Orion 13.5 in my touring car with the Mamba in and it worked great. Had a slight hesitation the first time I hit the throttle but after that it worked flawlessly.
I ran my 13.5 Orion with 4200's at the track I race at against everyone else and they all use 13.5 LRP or Novak 13.5 with Lipos and other than a little down the straight I was faster everywhere else. I'm still also 2 to 4 teeth off on the gearing as the motor is only 130 degrees after a 6 minute race.