Gear Ratio
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Junior User
Joined: May 2005
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From: Clovis,CA.
Gear Ratio
I am looking at putting larger tires on my truck. I currently have 3.50 gears with 33's. I am looking to buy and install a dana 70 rear dif and a dana 60 front dif. I want either 38.5 boggers or 39.5 iroks. Which gear ratio would be best. I do not see much hiway use so 55 mph would be my top speed. I hve a np435 manual trans . I mostly drive on trails not much rock crawling or muddin'
Postmaster
Joined: Jan 2005
Posts: 2,753
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From: Phenix City, AL
I posted this a while back. Hope it helps you out.
BFR250
Just though I'd put this up. I was trying to figure my effective gear ratio after changing tires. Did some stumbling around and found a formula for it. I know it was driving me nuts figuring it out, so here it is for anybody else trying to figure it ou.
(Original Tire Dia / New Tire Dia) x Current Gear Ratio = Effective Gear Ratio
Just change the gear ratio number to figure out what gears you need to install to get the effective ratio you want.
BFR250
Just though I'd put this up. I was trying to figure my effective gear ratio after changing tires. Did some stumbling around and found a formula for it. I know it was driving me nuts figuring it out, so here it is for anybody else trying to figure it ou.
(Original Tire Dia / New Tire Dia) x Current Gear Ratio = Effective Gear Ratio
Just change the gear ratio number to figure out what gears you need to install to get the effective ratio you want.
Post Fiend
Joined: Mar 2005
Posts: 8,542
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From: Louisiana
I wouldn't go any smaller(numatically) then 4.56, but 4.88 would prolly be the best one to go with. A good rule of thumb is to multiply any tire size by .12 and get a close gear ratio.
Like 39.5x.12=4.74
The higher gear ratio(numatically) the better your truck would perform off road, but I wouldn't go any higher(numatically) than 5.13 gears with very little road use.
Good luck.
Like 39.5x.12=4.74
The higher gear ratio(numatically) the better your truck would perform off road, but I wouldn't go any higher(numatically) than 5.13 gears with very little road use.
Good luck.
Lead Driver
Joined: May 2000
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From: Senoia, GA
i run 39.5" iroks with 4.10 gears, C6 auto, and NP205. my gearing isn't near what it sdhould be for rock crawling but i get some pretty decent wheel speed! i havent really had any issues with gearing so far but plan on bumping up to 5.13 gears and 205/205 doubler sometime in the future!
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From: Houston, Texas
Originally Posted by MBBFord
A good rule of thumb is to multiply any tire size by .12 and get a close gear ratio.
By your chart, my jeep needs 3.96 gears with my larger tires.....it came with 4.10's stock. It now has 488's and I am still not happy.
Hotshot
Joined: Dec 2003
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From: iowa
Jeremy that would be for street use, and reasonable rpm at hyway speeds. If I backwards calculate your figures you have 33" tires, and with 4.88 gears you will be turning 3230 rpm at 65mph which is really high rpms for any thoughts of reasonable fuel economy, and would actually be pretty hard on the engine. I personally wouldn't go with 4.88 gears until I was running atleast a 36" tire unless it's a dedicated trail rig.
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From: Clovis,CA.
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Posting Guru
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From: Houston, Texas
Originally Posted by monsterbaby
Jeremy that would be for street use, and reasonable rpm at hyway speeds. If I backwards calculate your figures you have 33" tires, and with 4.88 gears you will be turning 3230 rpm at 65mph which is really high rpms for any thoughts of reasonable fuel economy, and would actually be pretty hard on the engine. I personally wouldn't go with 4.88 gears until I was running atleast a 36" tire unless it's a dedicated trail rig.
You are correct, I have 33" tires.....but I turn 3000 rpm at 70 mph on the freeway. Perfect setup with the AX-5 transmission.
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From: Reed Point, MT
Originally Posted by Broke_Okie
So 4.88 or 5.13 would be good for my application. I heard 5.13 get smaller on d60's and weak
I guess if you think about it, the less power you have, the lower ratio you can use, however, if you have the power, you don't need the lower gears.
Cody
Last edited by cleatus12r; Aug 4, 2005 at 01:38 PM.
Its Comin Right for us!
Joined: Feb 2005
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From: GA
i dont know if i missed it, bu thow about telling us what truck/engine/tranny you are running?? having overdrive makes a huge difference
the stuff about lower ratios being weak only really applies to dana 44's and smaller. with 5.38's and down on a dana 60 you dont have to worry about anything if you know what your doing setting up the gears. and with dana 44's geared that low...u usually just blow up a shaft first anyway
the stuff about lower ratios being weak only really applies to dana 44's and smaller. with 5.38's and down on a dana 60 you dont have to worry about anything if you know what your doing setting up the gears. and with dana 44's geared that low...u usually just blow up a shaft first anyway
Elder User
Joined: Feb 2004
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From: Longvalley, SD
I am running 42's with a 466, 4.10s, dana 24, and np 435. Like fishy I am no way a rockcrawler but definitly get some wheel speed. I can do pry 70 mph in 3rd gear 2 hi. I have enough power to bang through the gears pretty fast but I will be going to 5.38's in the future so I can use high range in more mudding/wheeling situations.
Posting Guru
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From: South Dakota
I have a buddy who runs 4.56s, and 38" TSL Radials with around 425hp, and it does great, and his does it all from rocks to mud to snow.
So, like all the others are saying, we need to know some more info on your motor, tranny, t-case, etc....
So, like all the others are saying, we need to know some more info on your motor, tranny, t-case, etc....
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Junior User
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From: Clovis,CA.
OK ...I have a stock 390 FE engine/NP435 transmission/Dana 21- 1:1 single speed transfer case/33" mud tires/3.50 ring and pinion/difs do not matter because I am swapping in D70 rear and D60 front.Also soon to have a NP205 transfer case married to the NP435 transmission.