Topic: Gears... a quick overview

[Jeffy]

This has come up several times on the FB page as well as in the forum so I'll give me 2 cents.

First you've got to ask yourself, where are you having problems.  Is it onroad or off?  For onroad you'll want to look at your axle gears.  When installing larger tires from stock, you effectively change the ratio to a higher set of gears. 

Onroad:

In stock form, your stock tires were probably 27" (205.75R15, aka: base stock tires) and then you change them to 33''s.  Those stock 4.10's are now are equivalent to 3.35's.  To get the gearing back to stock you would need 5.01:1.  Since they do not make 5.01's, and to compensate for the extra weight of the tires you'd probably go with 5.13:1.  This will give you close to the same RPM as you would have at a given speed as stock.  It however doesn't account for the decrease in aerodynamics.

Offroad:

Now, your final crawl ratio would be the combination of the transmission gear x transfer case gear x axle gear.  In this case, in first gear (3.92), 4-Low (2.72) and stock axle gears (4.11), you would have a final ratio of 43.8:1.  Now if we changed the axle ratio to 5.13:1 the final ratio changes to 54.7:1.  Not that much of a difference. 

If you change the transmission to something like a SM420 (7.05:1) , SM465 (6.55:1), NP435 (6.681:), etc.. this would give you a final crawl ratio of 78.8 to 1, 73 to 1 and 74.5 to 1 respectively.  Keep in mind that these are 4-speed transmissions so for a daily driver, they aren't going to be optimal with a non-syncro'ed first and a huge jump to second and no overdrive.  Most people who go this route are doing it in conjunction with transfer case gears and or are towing their Jeep to the trail.


For most people I recommend looking for lower gears in the transfer case.   Options would be a Rubicon 241OR, TeraFlex's 4to1, Atlas 2, doubler boxes: like the KluneV, Rooster, Blackbox, etc.., or go all out and get an Atlas 4.


Again stock Jeep with a Teraflex 4to1 (3.94 x 4 x 4.10) would have a final ratio of 64.7:1.  You can already see this makes a huge difference.  With 5.13 axle gears that jumps to 80:1 which is very respectable.

Now with the doubler boxes you can have some extremely low gears which are great for the 2.5L.

Again a stock geared 2.5L with a 2.72:1 doubler would have (3.94x2.72x2.72,4.10) would have 119:1 final crawl ratio.  A better option would be to have them different ratios so you have more choices in your gearing.  The cheapest would be to keep the stock 2.72:1 in the transfer case and add a 4:1 KlineV David.  This would give you a 176:1 final crawl ratio.  The downside is that you'd need to beef up the stock transfer case with a Heavy duty output shaft and upgrade the chain and sprocket set from the 1" wide chain to the 1.25" chain on the Heavy Duty versions.

The best option though is the Atlas 4.  This is an Atlas 2 (4.3:1) with an integrated doubler (2.72:1) attached to it.   A stock 2.5L would have the stock 44:1 with just the doubler, 69:1 with just the main Atlas or a compound crawl ratio of 189:1.

Here's the cavit though.  If you need speed, ultra low gears aren't going to help.  Lower gears help by multiplying the torque.  With an engine that has 140 lbs-ft at peak, you have to gear lower than a V8 with 300 lbs-ft at peak.  Even off peak, they will have more than your peak torque.

tl:dr

Crawl Ratios at a glance:

Stock -44:1
Stock w/5.13 - 55:1
Stock w/Teraflex - 65:1
Stock w/SM465 - 73:1
Stock w/NP435 - 74.5:1
Stock w/SM420 - 79:1
Stock w/Teraflex & 5.13 - 80:1
Stock w/NP435 & TeraFlex - 107:1
Stock w/SM420 & TeraFlex - 110:1
Stock w/SM465 & TeraFlex - 115:1
Stock w/Doubler - 119:1
Stock w/Atlas 4 (4.3) - 44:1/69:1/189:1

Anyway, food for thought.

<a href="https://www.youtube.com/v/HmmXsX1auzc" target="_blank" rel="noopener noreferrer" class="bbc_link bbc_flash_disabled new_win">https://www.youtube.com/v/HmmXsX1auzc</a>

[chardrc]

Great New Thread. I would note that the SM420, SM465, and NV435 are actually 4 speed transmissions. In reality you will only use gears 2-4 unless you are off-road crawling as the low first gear is generally unneeded on the road. As Jeffy mentioned they are not great options for on road use with the lack of overdrive.

Another thing to keep in mind is how much wheel speed you want to maintain in reverse which will depend of the type of wheeling you do. If you wheel in the mud 4:1 low range can be a real hindrance to rocking back and forth with enough wheel speed unless you have the torque to use high range (A problem I have with my JK when I unexpectedly find mud). That being said if you stick to the rocks low gears are a great thing, Proto-2 has a SM465, Dana 18 with 3:15 teralow gears, and 5:13 gears in the axles for a crawl ratio of 105.8:1 with 35s which is great for crawling with only using 1st gear for real technical crawls but if you need to back up it is a slow process unless you jump to high range.

[Jeffy]

That's why I mentioned at the end a bit about wheel speed.  I think if you're wheeling in mud, the 2.5L isn't a good choice anyway.  That's why all of the Atlas Pro series only has 1.5:1, 2:1 and 3.8:1 so they have the wheel speed.  Lower gears are strictly a torque multiplier.  If you have more like with a V8 or diesel, you don't need as low gears.  It's also why Unimogs have extremely low gears since they have 100hp engines.