[Ramius]

What’s the difference between the “super” galaxy and the plain old regular galaxy? Bigger?

[El Gato]

Not bigger, same airframes, just rebuilt rather than newly built. Avionics modernized, with "glass cockpit". New, newly built that is, GE CF6-80C2 commercial engines that power many Boeing 747s, 767s, Airbus 300 and 310s, are part of the update.

Previous engine was the GE TF-39.

The new engines make them more fuel efficient, so they can fly further without refueling, or alternatively they can carry less fuel and more payload. (Although generally they are volume rather than weight limited) However the CF6 family of engines is derived from the TF-39.

Here's an article which talks about the upgrades that resulted in the C-5M.

The same engine will be used on the new Boeing 767 tanker... we hope, which will mean simplified logistics for the Air Force. Now if they'd just stick 4 of them on the BUFF...

[JRios1968]

What’s the difference between the “super” galaxy and the plain old regular galaxy? Bigger?

Bigger, more powerful engines, and a completely redone cockpit avionics suite...the "glass cockpit" is used extensively in the M model.

[AFreeBird]

Before and after pics of cockpit.

And a size perspective. KC-135 and C5

It looks like you could take the wings off a KC-135, and maybe cram two of them inside a Galaxy.