Hello,
The rule is clear enough. One computes the desired Engine Rating for a Superheavy 'Mech using the standard formula of Tonnage x Desired Walk MP. The Engine Tables then provide the weights for said engines. As with standard BattleMechs, the upper limit on speed is simply the biggest engine you can effectively mount.
Hopefully, that helps.
Thank you,
- Herbert Beas
BattleTech
Catalyst Game Labs