For a start, DCC mode speed is directly related to the max speed in the engine spec file, and every "notch" is just a percentage of that value (caveat: it's not a linear correlation....).
The next useful parameters are the max-accel and max-decel values, that just change the time to reach the speed at the desired "notch".
Finally, the "notch" speed will ALWAYS be reached.
What will change is the grades or the consist weight: on a positive slope the acceleration will be slower, a heavy train will be slower to accelerate and so on.
BUT....the "notch" speed will always be the same at the end of the acceleration, no changes even in negative slopes.
That said, I grabbed a steam loco and made two tests in two different routes on a new session, and the result was the same: "notch" 30, speed 7 mph on a light consist.
So, unless you're using different Trainz versions (just a guess), or different locos with the same model but different engine specs, there is no way to have 6-7 mph differences in the same "notch" on the same loco in any different route.
Try a different loco, be careful to use the same everywhere, make a new session (that is, a clean start) for every route and make a more controlled test.
Or...tell us what loco and what routes you're using, we may try our own tests.
Luca
