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BlackRamHemi 03-25-2012 01:59 PM

Gear Swap time 4.10's or 4.56's?
I've been stuck with 3.55's for almost 2 years now in my 10' RCSB 4x4
and the time has finally come to upgrade!

I've been running 28" M/T DR's on 17" wheels at the track to drop my effective gear ratio to around 4.30:1
The 295/45R17 Mickey Thompson ET's (x4 for my 4x4) shaved 5 tenths off my quarter, and got me into the 13's (13.8/98.5)
...along with the basic intake/exhaust/tune mod's everybody runs.

Most recently I stepped up to an Edge Torque Converter that stalls at about 3000 rpm,
and dropped another 3 tenths off my 60' down from 2.1's to 1.8's but traction is dependant on perfect conditions/prep now.
My new headers just arrived this week (JBA coated shorties)
and before I go much further adding power it's time to address my traction issues due to my open rear diff and 3.55's
I'm getting to the point that I often need to use an AWD (4WD Auto mode) and 4 drag radials for traction.

Since a Locker will keep me from doing a 1 wheel peel,
I may as well upgrade my gears when the rear carrier comes out in favour of a Detroit Tru-Trac limited slip.
I've been doing some calculation,
and I "think" ideally I should be aiming for about 5800 RPM at the finish line in a 1:1 3rd gear. Right?
I'm currently running a Diablo 93 tune, shift points are maxed out 6200-6400.

So what do you think? 4.56's too low? are 4.10's a better choice?
For daily driving I roll heavy 305/50R20 Terra Grappler's on stock 20's
90% city commuting and very rarely see any highway time.
I've recently found some good on-line calculators here...
Not sure what number to use for torque converter slip so I just used 10%

Here's where I'm at currently with 3.55's on my 28" tires:
3rd gear is falling on its face at 4600 rpm at the traps
with 4.10's put me at 5300, much closer to being in the powerband at the traps
and 4.56's put me right on the edge before 4th gear kicks in after crossing the 1320' mark
Anybody here running lower gears for racing confirm those numbers?
That last thing I want to do is end up shifting into 4th (overdrive)because I've gone too low with 4.56's.
I have to regear both front and rear diffs,so I need to get it right the first time.
I suppose if I end up with too short a gear,
to correct it I could just get new taller tire 30"-31" tires and sell the 28's.

FWIW, my best times at the track on my heavy Grapplers/20's are 14.4's with 3.55's
I bet I could get high 13's on them on 4.56's and my new headers :D

moparornocar72 03-25-2012 06:22 PM

By the calculations above it seems the 4.10's would be the best way to go but I'm new to all of this as well and have been trying to learn on the fly. I have the 3.92 and am at the top of 3 right before the 1/4 line but I have not adjusted the shift point yet. My best time is on the stock 20's and goodyears was 14.465 at 97 mph uncorrected. I have also been thinking of finding a set of m/t's. Hope you get it right!

KarateManAZ 03-25-2012 06:24 PM

90% city = 4.56's! How tall are your street tires?

InDeepWaterBlue 03-25-2012 08:23 PM

Not to jack your thread, but where can you get a locking diff and 4.10 gears at (currently using 3.55) at to prevent the one wheel wonders. I thought about having the R/T rear end put on but it doesn't seem very cost effective.

BlackRamHemi 03-25-2012 09:45 PM

My street tires are 305/50R20 Terra Grapplers
they are 12.2" wide by 32.2" high and on mystock 20's weigh in at over 90lbs ea.

I currently run 14.4's on them at the track
..and thats at 2500'+ DA, would be high 13's at sea level.

I'll probably turn them at 2300+ rpm on the highway with 4.56's, but that wouldn't be very often.
My truck is primarily my city commuter/weekend warrior and wont see 5th gear very often.

I run a 93 octane tune all year round, never less than 94 octane fuel,
my throttle boost and shift points on my Diablo Trinity are maxed out.

Fuel economy has never been a concern, go fast or go home.

Although I've never re-geared a truck for the track,
I have done many Jeeps with 4.56's and ARB air lockers front and rear,
so I do know and appreciate the performance benefits of a gear swap.

I was a Jeep Jamboree Trailguide for almost 10 years
my last ride....built by myslef from the ground up except for the axles, I leave that to the pro's

MF2000 03-25-2012 10:10 PM

I think you should think about where you want to go next with your mods. You need to base your gears on your final goal. If you add a cam-n-heads and/or power adder into your equation, you'll likely run out of gear at the big end or cross the line at an RPM that is not optimal.

CHVYKLR 03-25-2012 10:17 PM

I would go with the 4.10s. do your M/Ts actually measure out at 28" or closer to 27" when aired down? Also, wouldn't your trap mph increase with a quicker E/T? If your E/Ts dropped to 13.4 you'd be crossing the traps around 101 and that puts your rpms up in the 5600-5700 area. Am I way off in my thinking?

BlackRamHemi 03-25-2012 10:34 PM

This is why I'm asking for "Real" results from someone that's done it rather than guessing or relying on calculators.

I've read that a gear reduction only effects your ET,
and traps speeds remain aproximately the same if all other parameters remain equal.

With the limited experience I have, I believe that to be true.

I also believe that regardless of power adders or internal upgrades, like cam/heads,
RPM at the finish line will remain constant with tire size x gear ratio x trans gear ratio.

I have a bud with an SRT10 Ram who runs 4.56's and M&H 390/40R17 DR's (29" Tall) and a 100 shot
so there's no way I'm going to run more power than him, so I cant understand how 4.56's could be "too much" gear for my Ram with substantially less power.

As I said before, should it be too low, although I cant imagine it
an easy fix is to just go to a taller drag radial, right?

CHVYKLR 03-25-2012 10:37 PM

yeah, if it was too much gear you def could go to a slightly taller tire.

BlackRamHemi 03-25-2012 10:46 PM

Tires are WAY cheaper to swap out than Gears if you dont get it right the first time.
we're talkin a couple hundred$ vs a couple thousand $$$ in labour charges alone.

Having a 4x4 makes it twice as expensive if you're not careful ;)

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