Hey!
I believe what you are looking for is in the TweenGMS 2 beta. Passing a duration as an array [100] will tell the tween to use a normalized duration.
TweenFire(id, "io", 0, true, 0, [100], "x", 0, 500);
That would have x update at 100 pixels per second. So, the example above would have an actual duration of 5 seconds.
However, using step timing will have it update a certain amount per step.
TweenFire(id, "io", 0, false, 0, [5], "x", 0, 500);
The example above would move x by 5 pixels (on average) per each step/frame.
If possible, I'd take a look at the beta and see if this infact is what you are looking for.
-Stephen