Mach3 Problems problems problems.

[JAZZCNC]

It doesn't matter if you use inch or mm for setting up your motor steps per, if you've done it right the machine will run imperial or metric code accurately.
.
The body of each feature in the code is simply in numeric units and G20 or G21 in the header tells Mach 3 if those units will be interpreted as (G21) mm or (G20) inches.

.
Not exactly correct as explained above.!! . . . It will scale the moves but not the feedrate.
Try it load road runner and put G20 on first line or type G20 in MDI before running.
The Feedrate in Gcode will be in inches minute but motors tuning is relative to MM/min. This is why you have to retune the motors if you change from metric to imperial units.