We are currently building our autonomous program, and we need to understand the speed of our motors. We do not have a way to test our robot, so we have to figure everything out mathematically.
Thank you,
Octobots
You might have a look at this:
https://vexforum.com/t/motor-torque-speed-curves-rev2/21868/1
However, nothing is a good substitute for lots of testing. 
Long story short
Vex 393’s at 7.2v, spinning with no load is:
Default internal gearing :100rpm
high speed gearing :160rpm
turbo gearing :200rpm
However, the batteries are usually charged a couple volts higher than 7.2v, and thus the motor rpm is higher. Additionally, when the motor is under load, depending on how much work it has to do the rpm will drop. To get the rpm drop you’ll have to use the chart that FullMetalMentor provided in a link to a thread by jpearman.
fyi, The “Turbo Gearing” has a free spinning rpm of 240
Save yourself the trouble and just use encoders. I can speak from personal experience that relying on just the battery level will kill your autonomous almost every time. This was hours of tuning timings last year compared to this year of actually using sensors, resulting autonomous working 85% of the time compared to about 25% last year. (Skills was even worse, with only about a 5% perfection rate)