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As you stated you have a two-wheeled robot your kinematics looks something like this:

vth = (v1 - v2)/l;
vx = (v1 + v2)/2;
vy = 0;

With v1 and v2 being the respective wheel speeds and the x direction being defined as "forward" and l the distance between the wheels.

To get from the counts/s of the encoder to actual speed in m/s simply multiply it by the circumference of the wheel and divide by the counts of the encoder per wheel-revolution.