Gearing

[Dan]

Hey! So I know there's another thread about gearing in this section but I didn't want to hijack that post so here I am starting a new one...

I need some help with figuring out what to do with my gearing, I have been struggling with the logic behind what different changes do... Ok so I'll explain what I'm thinking and hope for some constructive criticism!

Ok so I have factory 3.55 gears with 38" tires and (soon to be) 75mm/94mm compounds so that being said, fairly large turbos to get lit. Now I know a turbo charged engine needs load to light turbos and when I had the single 68mm, the turbo didn't even light till second gear. Now I'm going to mess this up but if I gear **higher**?? (Don't know if the term is lower or higher, but putting 4.10 or a higher ratio than 3.55) that means that my first gear is going to be even more useless as far as turbos lighting... Oh the joys of having a almost 4:1 first gear!

Now in my mind it's a catch 22...

Because having 3.55s with the bigger tires I'm not at all in the sweet spot on the highway.. Especially with that single turbo and now even more so (I anticipate) with the twins.. So I got my paper and pen out and the axle conversion online..

At 75 mph, with 3.55s and the 38s in 6th gear I'm revving 1575 rpm.
At 75 mph, with 3.55s and the 38s in 5th gear I'm revving 2025 rpm.

So now,

At 75 mph, with 4.10s in 6th rpm is at 1822
At 75 mph, with 4.10s in 5th rpm is at 2340

I think with the 4.10s at 1800 rpm that engine will make enough torque to pull up hills without downshifting.. That was my biggest issue was instead of torquing up hills at 1600rpm (cruise on) the 68 wouldn't light and just smoke untill it finally down shifts and then sits you back in the seat till it gets back up to speed then up shifts again and smokes, down shifts... Hunting for a gear till the top of the hill (Might have been a tuning issue I don't know)

I'm looking to improve my drivability with gearing and yeah I know it's gunna be tough with such a big turbo combo but here's what I have running around my head: I could stick with the 3.55s, and that will load up the turbos quite well but I would be counting on the high stall converter to get things moving off the line. And on the highway just run it in 5th and then at 2000rpm there will be no issue keeping it lit!

Or

Go to 4.10s and hope that 1800 rpm is enough to keep the turbos lit on the highway cause if the truck downshifts, well 2350 is decently high.. And as far as getting things going when racing I could look into starting in 2nd gear to load up them turbos..

I know I'm overthinking this! Just trying to understand! Thanks for any input!

 

[Dzchey21]

410 will make 1st gear more usable
410 will make you turn higher rpm on the highway

 

[co04cobra]

410 will make 1st gear more usable
410 will make you turn higher rpm on the highway

:whs: 4.10 is gearing lower.

Unless you cruise at or above 75 all the time. 4.10s should make for a much more driver friendly truck with 38s. IMO.

 

[RedRice589]

With 35's on a 6.4 I was running a little higher on the interstate but it made driving so much nicer in town and actually all around for sure.

 

[4EverBoosted1]

You should be fine with 4.10s. I actually am running 3.30s in mine and I never use 6th gear unless I am going over 75.

 

[gloan93]

My truck has 3:31's hate it lol

 

[MadDiesel73]

yea im thinking about 3.73.s with these 35's waiting for mike to get it done see what he thinks.

 

[Dan]

Thanks for the help guys!!

:whs: 4.10 is gearing lower.

Unless you cruise at or above 75 all the time. 4.10s should make for a much more driver friendly truck with 38s. IMO.

Ok I'm curious why you say this? I mostly drive highway (90%), at 130-140 kph (80mph+) especially when I get the privilege to be on you folks Interstates!! Now with my choice of turbos, my peak torque is going to be higher than say a stock turbo, and if I'm understanding it correctly, best Milage, best drivability is at peak torque rpm range...

I might be way over thinking this and way ahead of myself! Hahaha maybe I should be on a dyno after it's all together, see what rpm peak torque is at and then choose my gearing to place my engine speed at that rpm at 75-80 mph kinda deal!


If that makes any sense!! Hahaha

 

[MadDiesel73]

What rpms you turning at 80mph?

 

[Dan]

What rpms you turning at 80mph?

1680 rpm @ 80mph

 

[MadDiesel73]

Id say you would want to be around 1900 at 80 to make an all round better driver. 4.10 would be perfect.

 

[Jason]

Off topic..and I know it's something completely different. But..a buddy of mine has a new ram..rcsb, with the bigger hemi..he did all the bolt ons and a good sized turbo..it is a s-frame unit..I wanna say close to what a s366 is. Anyhow..his truck had 4.30 gears and he could never get the turbo to light until the converter was locked around 80mph..then he'd see 4psi. Dropped gearing down to whatever was stock..and it lights much quicker, with way more..like damn near 3x the boost. Completely different animal.

In your case, the gearing would help make first gear closer to factory ratio..maybe a tad higher, so the truck doesntstrugle to get moving..and the gearing shluld help you all around. The long gears with your big tires now, I think makes everything work harder and less efficiently, because even though the turbo requires load to light..too much load can snuff it all out.