LS Swap Question

[DBear]

About to do a LS swap in a 78 k30 crew cab shortened with the following drivetrain: 2002 6.0/4L80e/np205, Dana 60F/14bolt rear.. Plan on running about a 33 tall and skinny tire.

Trying to figure out best axle gears, to have optimal room's st 70mph cruising speed. Just not sure what this motor/trans likes?

Thank you for any help you can provide

[Richard]

I typically shoot for about 2k at 70 in OD for a good compromise on performance and comfortable highway rpm. 4L80e with 4.10's will be just under 2k on 33's and going to 3.73's will lower rpm by about 150. Doubt there would be any gas milage difference between the two gears. 3.42's will lower rpm further. In my experience will hurt performance and gas mileage will not increase, may decrease. Calculations are at 70 with converter locked up, unlocked converter will jump rpms's about 200.

[mongocanfly]

What gears do you have now?
My C30 with 4.10s turns just over 2k at 65mph..I like it there..
On the IH I'm building ,I went with 4.56s and a 255/85-16 tire..should put it in the same rpm range as the C30..
for the next one I'm building, I already have a set of axles with 3.73s..it will also get the 255/85-16s....I'm going to try them and see how I like the 3.73s before I decide to do any changes..
all these trucks have/will have LS power and manual 5spds.....

[DBear]

Originally Posted by mongocanfly

What gears do you have now?
My C30 with 4.10s turns just over 2k at 65mph..I like it there..
On the IH I'm building ,I went with 4.56s and a 255/85-16 tire..should put it in the same rpm range as the C30..
for the next one I'm building, I already have a set of axles with 3.73s..it will also get the 255/85-16s....I'm going to try them and see how I like the 3.73s before I decide to do any changes..
all these trucks have/will have LS power and manual 5spds.....

Right now, I believe they are 4.56's, which there is no way I will run these. I am thinking of going to 3.73's. I will also be running 255/85/16's or something close to them. Thank you for the reply sir.

[mongocanfly]

if you have military axles, they should be 4.56s

using a rpm calc
a 255/85-16 is just over 33in tall
your od will be .7
at 65mph rpm should be 2113 with 4.56s
very close to what my C30 turns with 4.10s and a shorter tire

http://www.csgnetwork.com/multirpmcalc.html

but i think with 4.10s or 3.73s you should be ok as well

[DBear]

Originally Posted by mongocanfly

if you have military axles, they should be 4.56s

using a rpm calc
a 255/85-16 is just over 33in tall
your od will be .7
at 65mph rpm should be 2113 with 4.56s
very close to what my C30 turns with 4.10s and a shorter tire

http://www.csgnetwork.com/multirpmcalc.html

but i think with 4.10s or 3.73s you should be ok as well

It is a 78 K30 crew cab, not the military variety. Still surprised that low of gears in a civilian truck, but I guess that is what the Original owner wanted. I think I will re-gear it to 3.73. Now, just to figure out what brand of gears to go with, seems to be quite of a variety.

[Richard]

Bought my 94 with 3.42's (4l60e) because I lowered it and installed 315/40/17 tires. Got tired of that and lifted it. Installed a skinny 32" tire. Was a bit sluggish after that and went in and out of OD a lot. Not to long ago installed the 3.73's because pinion broke. Better performance and the extra 150 rpm is not even noticed on the highway (no change in milage). Almost wish I had gone with the 4.10s. Personally, I would install 4.10's with your scenario, will be more like my current setup. 3.73's will be about 1800 rpm with your tires. The 4L80 has a bigger gaps from OD to 3rd and 3rd to 2nd than the 4L60. So, avoiding the rpm increase with down shifts will make highway driving more comfortable.
Have used Yukon gears twice and had no problems yet. Have another set of 5.13 Yukon's I will install in the AAM 11.5 at the back of my 72 which will have 37" tires. LSA powered and a 6L90 trans.
Maybe try the 4.56 gears with the OD trans, you may be surprised. Only about 300 rpm at cruise over the 3.73 I would guess. You are driving a brick wall, so milage will probably not change with any gear.

[mongocanfly]

I'm on several gear setup sites..Yukon used to be top notch..doesn't seem that way now....revolution gear is the goto for most of the big time diff builders..
I agree with Richard, I'd try the 4.56s before you make your mind up...

[clay68c10]

You already own the 4.56s, so why not give em a try.
There's a great gear calculator here
http://www.grimmjeeper.com/gears.html

[DBear]

Originally Posted by Richard

Bought my 94 with 3.42's (4l60e) because I lowered it and installed 315/40/17 tires. Got tired of that and lifted it. Installed a skinny 32" tire. Was a bit sluggish after that and went in and out of OD a lot. Not to long ago installed the 3.73's because pinion broke. Better performance and the extra 150 rpm is not even noticed on the highway (no change in milage). Almost wish I had gone with the 4.10s. Personally, I would install 4.10's with your scenario, will be more like my current setup. 3.73's will be about 1800 rpm with your tires. The 4L80 has a bigger gaps from OD to 3rd and 3rd to 2nd than the 4L60. So, avoiding the rpm increase with down shifts will make highway driving more comfortable.
Have used Yukon gears twice and had no problems yet. Have another set of 5.13 Yukon's I will install in the AAM 11.5 at the back of my 72 which will have 37" tires. LSA powered and a 6L90 trans.
Maybe try the 4.56 gears with the OD trans, you may be surprised. Only about 300 rpm at cruise over the 3.73 I would guess. You are driving a brick wall, so milage will probably not change with any gear.

Just looked in my bone pile LOL, and I do have a 14 bolt with 4.10's with the correct spring pad spacing for my K30. I think I will go with the 4.10's like you are suggesting, and will only have to re-gear the front Dana 60.