66 GMC rear axle ?

[noJeepshere]

My brother is looking at a 66 GMC Pickup for his company billboard. He's wondering what axle under the rear of it. As far we know, it's a 1/2 2wd, with the 305 V6 engine. It originally came with a 3 on the tree, but now has a 4 speed, either the SM420 or 465, not sure yet. The current owner says it gets between 8-10 mpgs, so I think it has maybe 4.10's for trailer towing, which was it's use.

I wonder if mpg's could be increased by swapping in higher gears, maybe like 3.07 or 3.42's.

Thoughts?

 

[thatK30guy]

Rear should be an Eaton model, 6 lug, and round when you look at it from the rear. If you've seen the 3/4 ton Eaton rears in the pre-'73 trucks, it will look like that.

 

[b454rat]

Probly an Eaton like mentioned above, but GMC had to be different back then, so they might have used a Dana 44. I know in the 3/4 and 1 tons they used a 60 out back.

Look to see if the cover is round, Eaton, or octagonal, Dana 44/60.