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Does anybody know what the gearing on a beast like this is? It's a 1976 Grumman and has a dually rear end. It has a 390 and a c-6 auto trans. It's underpowered with the 390 and I'm toying with the idea of putting my Cummins in it. It's huge inside and has shelving and a work bench so it's a perfect rolling shop. Well..... it would be perfect with the Cummins in it. I'm just trying to figure out if I could use the Cummins with the C-6 and still have good highway speed and milage. Any info would be appreciated. Oh and by the way that 72 series 3 Land Rover is a possible 4BT project for the future. One project at a time.

 

Are you able to get a floor jack under it and pick the one side of the rear end off the ground? If you can, then chock the front tires, put the trans in neutral, mark the driveshaft, and turn the TIRE 10 times while counting the number of driveshaft rotations. This is much easier if you and a buddy to help you count. So if you spin it and get 31 driveshaft rotations then you most likely have 3.10's, ~35 - 3.55's, 41 - 4.10s. Hope this makes sense.

Being that thats a delivery truck I would say it has 4.10s in it, maybe even 4.55's.

 

Thanks for the info . If it's 4:10 I think I could squeeze out 60-65 on the highyway. Something I hadn't considered is how to get the c-6 to shift without vacuum modulation. Maybe with a few new parts my getrag would be easier.

 

If it is a Ford chasis it might even have 513 gears. Easily changed to 410 if you wanted to. The c6 will be what governs your speed. In a F350 and 410 rears the 7.3 diesels would only go about 65 redlined.

 

just talked to a fella running svo in his step van yesterday . his van had a 4bt cummins repower and a four speed. with 16 inch tires. has 3.73 with an additional od now it will cruise at 65mph without screaming. drove it coast to coast on veggie oil.

 

I have a 76 GMC step van. It has 4.10 rear end, and will run past 75 with no head wind, and of course with a 400 sb chevy. I bet yours is lower like 4.56.
I also have a 87 with the 4BT ( old bread truck) and it has 3.73's.

 

Mine has the 18 inch wheels. I climbed under the beast and looked at the axle. It's a Rockwell. The tag inside the truck says that the real axle is rated at 9080 lbs. I haven't figured out the gearing yet. Guess that I need to get a buddy over to help me count the revolutions.