3.21 was the standard ratio for us in CA thru 1979. the 3.55 axle ratio was optional, as was the sure grip. All of the 77 machos have had the 9 1/4 axle in them, and the 78's have been a mix of the 8 3/8 and 9 1/4. If the truck was ordered with the trailering equipment, then it got 3.55 or 3.91....although the 3.91 was not available of the 440....even a 440 macho could have gotten the 3.21 axle ratio with the small 8 3/8 rear. I had a nice black/blue adv se 78 power wagon that had 3.55 with SG and it was a 9 1/4 rear...
All ramchargers came with the 9 1/4 rear...

It would make sense that Machos would have be geared 3.55 since they came with the larger 10-15 tires.

Thanks for replying. Now, is there any way to find out my ratio?, cause I'd really like to know it. My info includes 78 macho built in november of 77'. It's got the 9 1/4 rear and i dont know if its suregrip. My guess is it's most likely 3:55 because of the larger rear 9 1/4. What I wanted to know from the beginning was if you could have the larger rear axle with 3:21 gears, and i guess you can.

You could make a chalk mark straight across the shaft onto the rear differential, jack it up and spin the tire one full turn and count how many times the drive shaft turned. If the shaft went 3 & 1/4 turns it's a 3.21, 3 & 1/2 turns it would be a 3.55 and if the other side tire turns opposite then it would be an open differential.