TrxR

Me and a budy are contemplating a building a trail truck using an old Dakota frame with a full cage and a bunch of reinforcements. The truck is going to ride on a set of 2.5ton Rockwells and a 4 link front and rear. My question is we were suggested to use a 12V cummins from a friend with a 4speed auto. Now my question is will those rockwells be too low sped for the Cummins? The tires will be somwhere between 46 and 53 inch as we are thinking about either using Military or tractor tires. We also though about using Bio diesel if we went with the cummins.

Thanks

bj-666

this question is unanswerable we need to know the ratio of the transfer case, the speed you want to run at, the gear ration of the rockwells, and the gear ratio of the tranny, then we have a starting point

Dartmouth 12V

As bj-666 said, you can make it work by choosing the proper ratios throughout the drivetrain. Since you say offroad only, how fast does it need to go? Is it going to be trailered or do you need to be able to go at least 60mph. The other way to look at it would be your desired crawl speed. I don't think that it should be too hard to make it work as long as you put the time in ahead and match the components before you go buying them. There are several good online calculators where you can input your gear ratios and tire size and get a chart of engine rpms versus road speed.

LanceD

u could always get a gear vendors overdrive too.

sounds like a solid project, can't wait to see it finished!

Jreif80

I believe all rockwells came with something like 7.54 or 7.14 anyways 7.somethings with the size of tires you are thinking of using it should be decent though i would still do your math to find out your over all crawl ration. now as a trail rig do you want a rock crawler that can Idle over everything or do you want something to use a little speed with? and actually a 12 valve sounds wonderfull but I've thought about making a diesel rig and I would use a cummins 4b but that is to my taste I am not sure what you would like. sounds like a pretty cool project though keep us posted

mr_ed

6.72 and that's it. No aftermarket re-gear for a Rockwell.

I seen a video on youtube of a guy with a 4bt rock buggy. He had some pretty good size meats and it was turning them ok...real low gearing though. Not as heavy as the 6bt neither, which is worth alot off road. Of course if you're gonna mud bog he's gonna need all the power he can git to turn those big tires, probably more than a 4bt could make.

Good luck though! Can't wait to see it! <-- beer, not coffee

displacedtexan

What are you planning on wheeling on?

Ru rammin 2?

Right on on the gear ratio. My brother's truck was running 44 inch swampers with 5.88 gears and they were just about like 4.10s with 235/85/16s. It is a
'77 f-250 and we upgraded to 46'' baja claws, a 10:1 460, and a c6 auto but putt a 70 with a locker and 4:10s in the rear instead of the 60 and the 400/4spd. It runs fine. motor is a factory ford high compression ambulence 'bout 400 horse and it does pretty good. Roasts the tires. But the 5:88s were fine not too low the 60 wasn't heavy enough. We run a factory np205, 2:1 gear reduction, and the ratios are pretty good. No rock crawling, just a big truck, but I bet it would have done fine gear and power wise with the 5:88s it mud bogs pretty good. Run a 2:1 205 and probably some 46-52'' meats and it should work if you've got the power. even with low gears, it takes some power to move those big tires.

TrxR

Sorry about the missing info. The rocks have a 6.72 gear as been said. Was going to use a 4spd auto that comes behind the Cummins and the transfer case that comes with it. But if I found a 2wd donor we were going to use the divorced 205 I have sitting in the corner. This is going to be used mainly on the trail and mud use and will be the main recovery vehicle for all my buddies. My other drive train option is the 400/727/203 thats not being used at the moment. It would be nice to be able to run at 50-60mph at times for some limited road travel (trail to trail) nut not totally necessary.