So I am trying to figure out what route to go w/ my CUCV. I had been towing my trail rig with our 3/4 ton 454 burban till my wife traded it in. Now I need to use my 86 CUCV to tow with. I have a single axle trailer (was made for a ski doo) towing my samurai. its a pretty light setup. Right now I have stock axles under it. But in my garage i have a 14 bolt with 3:42's in it, and a 14 with 4:10's. I am trying to decide which one to swap in, i want to run 33-35 inch tires. I am swapping out the TH400 for a sm465, and putting a turbo on it. Any recommendations would be greatly appreciated.
CK5
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Question about towing with my CUCV
- Thread starter us74k5
- Start date
I had a sub with 3.73 and 33" tires and a turbo 6.2 it was quite peppy but when with a load I felt it could do better, it did have the 700R4 though.
I would still go 4.10 given your other choice is 3.42 which is not good enough for the towing.
If you go with the 4.10 you could do fine with the 35" tires and 465 tranny
Actually, the stock 4.56 gears and some 37" hummer radials would be easier and about the right combo. I know a guy in Tacoma that was selling the 37's with 90% tread for $125 ea. But, I have a set of 4.10 geared 3/4 ton axles I'd trade for your one ton axles 
(completely rebuilt 8 lug 10 bolt front axle and detroit/disc'd 14FF).
(completely rebuilt 8 lug 10 bolt front axle and detroit/disc'd 14FF).
MaxPF
1/2 ton status
Go 3.42's. The governor on a 6.2 quits at ~3600, and you are defueling by 3300-3400. With 4.10's, you will be turning 2550 at 65MPH, which is going to hurt fuel economy, and yet you cannot downshift until you drop down to 50MPH, where you will sit on the governor at 3350-3400RPM. 4.56's are even worse. You will turn 2850 at 65MPH, which is gonna blow through fuel like a gasser. You cannot downshift until you drop down to 45MPH, where you will once again sit at 3350-3400RPM. BTW, I'm calculating these numbers with 35" tires. With 33's it's even worse 
With 3.42's you will cruise at a nice, leisurely 2130RPM at 65MPH. At 75 you will still be turning a relatively comfy 2460RPM. The good part is you can downshift to 3rd at 60MPH, so you won't be crying every time you hit a grade or want to pass. It may seem counter-intuitive, but the taller diff gears will actually give you better use of the engine's powerband at road speed.
Interestingly, 3.42's and and the 465's 1:1 top trans gear gives you the exact same cruise RPM as 4.56's and an NV4500's .75:1 OD. The NV4500-4.56 combo does allow a 5-4 downshift at anything up to 80MPH, but the resulting 25% reduction between 5 and 4 isn't usually enough for a pass or a long hill, and you absolutely cannot shift from 4th to 3rd until your speed drops to 48MPH or so.
I went through this whole exercise when determining gearing for my rig. I also have practical experience regarding just how bad 4.56's and a diesel suck, even with an OD trans. In my case I was running a 700, but 1st, 2nd, 3rd, and 4th are nearly identical to 2nd-5th on an NV4500. I had 4.56 gears in the axles and 37" tires, and it would turn 1900RPM at 65MPH. On minor grades (2-3%) I could pull 3rd gear and hold 65MPH. 6-7% grades were my Achilles heel. 3rd wasn't enough gear, but like the situation I mentioned above I couldn't downshift until around 48MPH. Then, I would simply sit on the governor doing 48MPH up the hill. This is in a 5900lb K5 with maybe another 700lbs of gear and driver, with a strong running 165HP J code 6.2 under the hood. I knew the engine had enough power to do 55-60MPH, but I simply didn't have the right gearing for it. Trying to upshift at 48-50MPH (3400-3450RPM) would drop me down to 2050RPM, which was too far off the engine's power curve to gain, or even hold, speed
Likewise, even if I hit the grade balls to the wall in 3rd my speed (and RPM) would continue to drop until I was forced to grab 2nd at 48MPH. 2nd gear was too short, and 3rd was too tall. In retrospect, I would have been better off with 4.10's and the OD trans, but then my cruise RPM at 65MPH would have dropped down to 1700RPM, which would have been on the back side of the torque curve. Still, that would have been a better compromise to have an extra 5-10MPH on the hills.
Anyway, enough of my diatribe. I'm sure many will disagree with me, and that's fine. The moral of the story is to match your diff gears to best utilize the powerband of the engine and the transmission's ratios. Simply going for lower gears is counterproductive - if you need off the line grunt, that's what granny low is for
With 3.42's you will cruise at a nice, leisurely 2130RPM at 65MPH. At 75 you will still be turning a relatively comfy 2460RPM. The good part is you can downshift to 3rd at 60MPH, so you won't be crying every time you hit a grade or want to pass. It may seem counter-intuitive, but the taller diff gears will actually give you better use of the engine's powerband at road speed.
Interestingly, 3.42's and and the 465's 1:1 top trans gear gives you the exact same cruise RPM as 4.56's and an NV4500's .75:1 OD. The NV4500-4.56 combo does allow a 5-4 downshift at anything up to 80MPH, but the resulting 25% reduction between 5 and 4 isn't usually enough for a pass or a long hill, and you absolutely cannot shift from 4th to 3rd until your speed drops to 48MPH or so.
I went through this whole exercise when determining gearing for my rig. I also have practical experience regarding just how bad 4.56's and a diesel suck, even with an OD trans. In my case I was running a 700, but 1st, 2nd, 3rd, and 4th are nearly identical to 2nd-5th on an NV4500. I had 4.56 gears in the axles and 37" tires, and it would turn 1900RPM at 65MPH. On minor grades (2-3%) I could pull 3rd gear and hold 65MPH. 6-7% grades were my Achilles heel. 3rd wasn't enough gear, but like the situation I mentioned above I couldn't downshift until around 48MPH. Then, I would simply sit on the governor doing 48MPH up the hill. This is in a 5900lb K5 with maybe another 700lbs of gear and driver, with a strong running 165HP J code 6.2 under the hood. I knew the engine had enough power to do 55-60MPH, but I simply didn't have the right gearing for it. Trying to upshift at 48-50MPH (3400-3450RPM) would drop me down to 2050RPM, which was too far off the engine's power curve to gain, or even hold, speed
Likewise, even if I hit the grade balls to the wall in 3rd my speed (and RPM) would continue to drop until I was forced to grab 2nd at 48MPH. 2nd gear was too short, and 3rd was too tall. In retrospect, I would have been better off with 4.10's and the OD trans, but then my cruise RPM at 65MPH would have dropped down to 1700RPM, which would have been on the back side of the torque curve. Still, that would have been a better compromise to have an extra 5-10MPH on the hills.Anyway, enough of my diatribe. I'm sure many will disagree with me, and that's fine. The moral of the story is to match your diff gears to best utilize the powerband of the engine and the transmission's ratios. Simply going for lower gears is counterproductive - if you need off the line grunt, that's what granny low is for
Max, I am running 37's and 4.10 gears and can't pull a 3% grade in 3rd gear (TH400) due to the lack of power. On flat ground it does ok, but I can't get out of my own way on the freeway... then again the motor has 270K on it now too so that is probably part of the problem. Hopefully a turbo and a fresh motor is all that it would take to fix that problem.
But, I used to drive a '79 K20 with a 6.2L, SM465, 4.10 gears and 33" tires. I could pull anything, anywhere. I just couldn't go over 60 mph on the highway
The best combo I could find so far (on paper) would be an SM465 with a ranger OD/gearsplitter. Being able to split the 3-4 shift would be the ticket for those long steep grades plus toss in an OD gear to make a lower gearset work better.
But, I used to drive a '79 K20 with a 6.2L, SM465, 4.10 gears and 33" tires. I could pull anything, anywhere. I just couldn't go over 60 mph on the highway
The best combo I could find so far (on paper) would be an SM465 with a ranger OD/gearsplitter. Being able to split the 3-4 shift would be the ticket for those long steep grades plus toss in an OD gear to make a lower gearset work better.
MaxPF
1/2 ton status
Well, to be fair I had a 165HP engine with only 60k on it, so I had a bit more oomph.
I also had 4.56's, so the engine was turning more R's. But, I used to drive a '79 K20 with a 6.2L, SM465, 4.10 gears and 33" tires. I could pull anything, anywhere. I just couldn't go over 60 mph on the highway
Exactly why 3.42's are better diff gears for the combo. You can still grab 3rd and have the power, but cruising in 4th gives you reasonable RPM's and the ability to do 80+ if you want.
I vote for the underdrive version of the ranger and 3.42's. Same result as above, but you aren't stirring 2 sets of gears while cruising. I have mentioned this before elsewhere, but it just isn't logical to crank your engine RPM up with an OD (which is lossy) just so you can massively reduce it again in the diff
It makes far more sense to simply pipe the engine torque straight through the trans with minimal loss, and use less gear in the diff to achieve the same outcome.
I vote for the underdrive version of the ranger and 3.42's. Same result as above, but you aren't stirring 2 sets of gears while cruising. I have mentioned this before elsewhere, but it just isn't logical to crank your engine RPM up with an OD (which is lossy) just so you can massively reduce it again in the diff
That would be better too...
Us74k5, do you have a matching front axle for the 3.42 14 bolt? If not I have an 8 lug 10 bolt with 3.42 gears over in the Tri-Cities. I was looking at running over there to pick up everything I have left at my friends house and take it down to the Portland Swap Meet to get rid of it all.
It would be easy to swap the 6 lug parts over and then you would be able to resell the 8 lug outers even.
