M551 MBT

There were several proposals for new designs based on the chassis of the M551 Sheridan. One of these was aimed at utilizing as many Sheridan components as possible to create a lightweight interim main battle tank: something that could be expedited until the MBT-70 entered production. It would have featured an elongated hull to allow for a smoother front glacis and heavier armor.

This tank probably would have taken advantage of a new gun and engine which were available at the time and already being considered as upgrades to the Sheridan. I can't find a suggested weight or armor values for the tank, so I've gone and left them blank. It would also probably be slower too, maybe around 56-64 kmh (or 35-40 mph).

Truthfully, the M551 MBT is a medium tank in name only. It's essentially just a heavier Sheridan, which sees some improved armor and gun handling to make up for the undoubtedly worse mobility.

Guns: 152 mm Gun-Launcher XM81E6 (conventional)

Engines: Continental LVCR-465 (450 hp)

Turrets (Turret Armor (mm)): ??/??/??

Suspensions/Alternate Hulls, Total Weight (metric tons): M551 MBT (??)

Hull Armor (mm): ??/??/??

Top Speed: ?? km/h

Crew: 4 (Driver; Gunner; Loader; Commander/Radio)

See Also: Sheridan, M551A1

Sources:

Hunnicutt, R. P. Sheridan, 2015. 

AD 821500, Engine-Transmission Power Plants for Tactical Vehicles