Legacy Speed Steps

Most engines are compatible, including diesel and steam, these should run the same speed or close. There is a small variance even between same product run A and B units though not like you describe.

Have seen a few where Lionel noted that they are not.

If the GP-7 doesn't start at the first speed step, see if the Odyssey is turned off.  It should run at exactly the same speed as the SD70ACE with the same speed step.

Weird.  As GRJ says, they should be the same.  Unless your SD70ACE was somehow entered into your CAB-2 as a "switcher" (in which case, speed steps are halved.)  Do you have a third engine to compare, to see which of the two is running "right?"

What happens if you place them both on the track (widely separated, with the GP-7 in front.)  Use the "standard railroad speeds" and set them both to medium speed, etc.  Do they even out after a while?

Thanks for the info, I should have mentioned in my original post that I did have both on the same track, and the GP7 would catch the SD70ACE or any other engine I own.  

I did check the odyssey switch and it is on, sounds like this unit needs to go in for repair. 

Is it possible that it thinks it's a TMCC locomotive?  Obviously, something is amiss, it could be an encoder issue.  If it's missing counts, it would go faster for a given setting.

Thanks, I was thinking that, that has something to do with the tape around the flywheel correct?

No tape in Legacy, it's a toothed wheel.