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Gearing ( opinion, theory, etc)
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Author:  65CrewCabPW [ Wed May 09, 2012 10:09 am ]
Post subject:  Gearing ( opinion, theory, etc)
My truck has 3.55 gears ( I believe) and with the weight being quite heavy, and quite tall (4x4), hauling itself up grades makes the slant sweat pretty bad... and shifting down with the NP435 results in... lots of noise and even slower speeds.

The gearing on the NP435 is 6.68, 3.34, 1.66, 1 which makes the jump between gears insane. Especially the 3-4 jump, which is really useless, in that on hills, 3rd is just winding the snot out of it and moving nowhere, shift, and it's lugging, to try to shove the truck along. Even worse if you're facing a headwind.

Yes, I use this truck off-road, but 1st almost always too slow to be useful, there's always low range, if you really need to go slow, which is insanely slow in low range, in 1st. No, I'm not into recreational rock crawling.

So, investigation says that an NP445 will bolt in place of my 435, and the ratios are 4.48, 2.28, 1.31, 1. Using a speed to rpm calculator, it shows what seems to be a lot more useful 3rd gear rpm to speed. As in you can shift on a hill and at 3000 rpm, be closer to 50 mph, maybe 55 with a little taller tires like I have. Oh, and it's got synchros in 1st gear! Very nice offroad.

Am I barking up the wrong tree here? Overlooking something? This seems like a lot better idea that changing to 4.10's and keeping the np435 I have now.

Any thoughts?
Author:  Mroldfart2u [ Wed May 09, 2012 10:32 am ]
Post subject:  Re: Gearing ( opinion, theory, etc)
Quote:
My truck has 3.55 gears ( I believe)
I would make for CERTAIN that is what you have.
Quote:

and with the weight being quite heavy, and quite tall (4x4), hauling itself up grades makes the slant sweat pretty bad... and shifting down with the NP435 results in... lots of noise and even slower speeds.
Yes, underpowered, AND under geared. (final drive ratio)
Quote:
The gearing on the NP435 is 6.68, 3.34, 1.66, 1 which makes the jump between gears insane. Especially the 3-4 jump, which is really useless, in that on hills, 3rd is just winding the snot out of it and moving nowhere, shift, and it's lugging, to try to shove the truck along. Even worse if you're facing a headwind.
Never have figured WHY engineers design/build with such great of ratio changes. (without being somekind of 'specialty' close ratio 4sps.. ie for muscle cars etc)
Quote:

Yes, I use this truck off-road, but 1st almost always too slow to be useful, there's always low range, if you really need to go slow, which is insanely slow in low range, in 1st. No, I'm not into recreational rock crawling.
Understandable, takes a 'special' kind of person to rock climb, and serious climbers usually build crawlers and leave the full size stuff for towing.
Quote:

So, investigation says that an NP445 will bolt in place of my 435, and the ratios are 4.48, 2.28, 1.31, 1. Using a speed to rpm calculator, it shows what seems to be a lot more useful 3rd gear rpm to speed. As in you can shift on a hill and at 3000 rpm, be closer to 50 mph, maybe 55 with a little taller tires like I have. Oh, and it's got synchros in 1st gear! Very nice offroad.
If its a direct bolt n go, I see no disadvantages. anything to help the spread is going to be beneficial. Not for sure what you are considering "taller tires" tho. And yes the syncro'ed low would be great for off road.
Quote:

Am I barking up the wrong tree here? Overlooking something? This seems like a lot better idea that changing to 4.10's and keeping the np435 I have now.

Any thoughts?
No you arent "barking up the wrong tree" But if it were me I would take the tire size into consideration. I wouldnt go as far as saying 4:10's AND new tranny would make the truck more pleasant, But I would really consider 3:90's range of rear end gearing, if you are going to keep present Tire sizes from here on out. If you ever go bigger tires again tho, you may have wished you had went to 4:10's.

M2C

( I have my own off-road 'toy' as it is, and its geared a little on the high side I think, but it works well enough its not worth the money to regear, without upgrading the housings them selves. ('83 Suburban 12-14" lift, 39.5 tires, 3:73's and the 700r4, sbc)
Author:  sandy in BC [ Wed May 09, 2012 11:02 am ]
Post subject: 
I too run a Suburban (1000000 miles in the last 30 years)

My latest (91) has an EFI, Vortec headed 350 with roller cam, headers etc. Likely in the 300-320 hp range.....mega torque

With 31" tires I run 4.10 gears for highway use . This with a 3 speed non OD Turbo 400. Iused to run 3.73 gears but found them too leggy for trailer towing or off-roading.

Gear so you are at maximum torque at your desired cruising speed.

ie: 3000 rpm at 60 mph

3.55 gears with 31" tires is way high geared. You should prolly have 4.56 with that size of truck and tire.
Author:  65CrewCabPW [ Wed May 09, 2012 11:54 am ]
Post subject: 
I run tires currently that the manufacturer specs as 30.2 in tall. Eventually, I plan on changing to tires that are 31 to 31.7 tall. The tire originally specified for this was 235/75-15, which are nominally 29 inch tall. I have 15 and 16 inch rims, all of which are 8 inch wide. The 265/75-16's I'd like to use list as 31.6 in. I could also use 31/1050-15's, which are ... 31 tall :)

I have driven the truck with 33 in tall tires ( 285/75-16) and that was seriously too tall of gears.

For highway use, the engine just loves what I have now, as it will cruise on the highway at up to 75 (about 3200 rpm?) very easily. Going shorter gears will mean noisy and slow highway trips.

And the fuel economy if you stay 60 and under is a factor, as well. I get about 14 on the highway, if you stay under 60. I expect I have some to gain there as well.

Really, with the gears I have, it runs out of oomph about 2200 and below, which means once it falls under 55, you're out of the engine's good power range. It runs strongest about 2600 to 3200.
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