Gear Swap time 4.10's or 4.56's?

18406 Views 81 Replies 18 Participants Last post by  HammerHead
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
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I'd go for more rpm than 5800 thru the traps. 6300 with you're tire dia and 100 mph would work out to 4.77's. I think 4.56's would be perfect and even if a little short you'd just have to put a taller tire which would look better anyway.
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