How do I figure out how fast my revo should go? Setup as of now close 2speed kit mach .26 motor runnin on 20% atm running 15/38 but wantin to switch to 17/36

I have a gps that straps to your wrist. Works great for me.

If you know the rpm of your engine you can get a rough estimate here

http://scriptasylum.com/rc_speed/top_speed.html

But yeah a gps strapped to the car would me the most accurate..