Brushless math
 

So I had a cc Sct 3800kv in my CRT.5. The esc broke so I bought a sv2 with 6900kv. To find rpm multiply the battery Volts by kv.

So if 3800x7.4 is 28120 and 7.4x6900 is 51000 why is the 6900 about the same speed as the 3800 with the same exact gearing?

Got me stumped

I would guess that with the same gearing the 6900kv motor is going to run HOT compared to the 3800. That's a big difference in RPM, the 3800 is going to make way more torque than the 6900, the 6900 simply isn't going to be able to "pull" that much gear. You're basically bogging it down and it can't spin to it's rated RPM. Put a smaller pinion on it and it should run faster..

Kv ratings are with no load on the motor, a motor with twice the Kv isn't going to be twice as fast. There are way to many factors involved.