So first of all, you want to make your entity move with actually metres (in quants that is) per real time second. The real time second is just a simple calculation via this line:
Code:

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time_step / 16;

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I would advice you to use time_smooth. If time_step changes suddenly, the speed becomes inaccurate during the next frame, adding to the noticeable jerk. This can be prevented by the time_smooth variable.

Code:

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(constant_speed * (metres_in_quants * (time_step / 16)));

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I guess this should do the trick. It's a simple theory but you could improve it using physics I guess. This exemple is in figure speech a 'poor man's physics' example

Cheers

Frazzle