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GMFS Enforcer
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Discussion Starter #1
Putting "Sticky" in the title is bold, but this information is needed. In an effort to curve or possibly stop the endless barrage of “What Gear?” threads that pop up daily on this page, I am providing some simple calculations to help in the gear selection process.

The Quick and Dirty:

Your have increased the size of your tire, what gears should you buy?

Old Tire Size – 31”
New Tire Size – 35”
Current Gear Ratio – 3.73

New Gear = (New Tire Size / Old Tire Size) * Current Gear Ratio

4.21 = (35/31) * 3.73

Since you cant buy 4.21 gears, it would be advised to purchase 4.56 gears. It is advised to round to the next lower ratio to account for increased rolling resistance, and reduced aerodynamics from being lifted.

Method 2:

This method is a bit more favorable as it takes into account a target RPM in your engine's power band for a given cruise speed.

New Tire Size – 35”
Target RPM – 2500 RPM
Trans. Overdrive Ratio – 0.70 (4L60E)
Highway Cruise Speed – 70 MPH
Correction Factor - 0.8

New Gear = (Target RPM * Tire Size)/(Cruise Speed * 420.38 * OD)

4.24 = (2500 RPM * 0.8 * 35”)/(70 MPH * 336.3 * 0.70)

In this instant you would use a 4.56 gear. Note that this calculation has a correction factor that provides a little insurance for passing, hills, towing, hauling, etc.. I used 0.8 which should be about right for most engines. If your engine makes A LOT of torque at your target RPM, you can use corrections factors of 0.7, 0.6, 0.5 etc... Also, this calculation assumes that a lockup stall converter is used in automatic transmission applications.

Here is a good link with some very good calculators. Bear in mind that no corrections are applied with these calculations.

http://www.bncoffroad.com/ratio/

EDIT: Method 2, see post 10.
 

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You're probably fighting a losing battle, but those of us that do use the search function will definately appreciate this lol. Great info man!
 

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Bayou Surface Drive, LLC
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You're probably fighting a losing battle, but those of us that do use the search function will definately appreciate this lol. Great info man!
When people ask about the gear ratios, link this thread. Simple

Great job Mr. Bryan!
 

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ΜΟΛΩΝ ΛΑΒΕ
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Going by method one which I originally used to get my gear ratio, I come up with 5.11, which comes out just right to my 5.13 ratio. (I originally chose 5.13s when I had 37s which gave me 4.97 ratio rounded up to 5.13)

Using method two...(4300RPM*38")/(70MPH*420.38*0.75)=7.4 :eek: I think I'll stick with method one....
Even with stock tires, method two yields a 5.94 ratio.
 

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Bayou Surface Drive, LLC
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Method #2 for my Dmax.... LOL at the numbers
Stock
(1600*30)/(70*420.38*.61) = 2.67:1

35"
(1600-35)/(70*420.38*.61) = 3.12:1
 

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GM bit-size owner
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Nice write up! Definately needs to be stickied. With these calculations, my ratio choice is just about perfect. It came out to 3.70 and I bought 3.73's, not quite as tall as I expected but it'll be a slight improvement.

edit: I know my bigger tire size isn't quite like you guys but 33"'s are all I need....for now. and it's a 2wd so no need for any bigger.
 

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GMFS Enforcer
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Discussion Starter #10
Using method two...(4300RPM*38")/(70MPH*420.38*0.75)=7.4 :eek: I think I'll stick with method one....
Even with stock tires, method two yields a 5.94 ratio.
Method #2 for my Dmax.... LOL at the numbers
Stock
(1600*30)/(70*420.38*.61) = 2.67:1

35"
(1600-35)/(70*420.38*.61) = 3.12:1
Wow... I see the flaw with Method 2 and have edited it. Instead of engine torque peak, I have used the term "Target RPM" so you can shoot for an RPM that is on the engines power band. I have also made it to where you can change the correction factor depending on how much torque your engine makes at the target RPM.
 

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just did mine.

(2200rpm * .8 * 35)/(70 MPH * 336.3 * 0.70) = 3.73 (what i have right now/came stock)

but i want 456!! lol i guess i need to do a higher rpm. plus i'm only at about 1900, 2000 rpm at 70 tho
 

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GMFS Enforcer
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Discussion Starter #12
Try a factor of 0.9 or 1. Also, see how much torque your engine makes at 2200 RPM compared to it's peak. If there is a big difference, it wouldn't hurt to go to a lower gear.
 

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GM bit-size owner
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1,654 Posts
Nice write up! Definately needs to be stickied. With these calculations, my ratio choice is just about perfect. It came out to 3.70 and I bought 3.73's, not quite as tall as I expected but it'll be a slight improvement.

edit: I know my bigger tire size isn't quite like you guys but 33"'s are all I need....for now. and it's a 2wd so no need for any bigger.
Dammit, just pulled the cover off to find that the 3.73 w/g80 rear I just bought is actually a 3.42 w/open carrier. WTF!
 

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this
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Ya know, I hate to be, "That guy" but you should also factor in your stall when selecting a gear. However, since most people don't bother to change their TC, then this thread is a pretty good way to determine gear ratios.


Also, I would like to add, there is NO advantage like mechanical advantage.
 

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GMFS Enforcer
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Discussion Starter #15
Ya know, I hate to be, "That guy" but you should also factor in your stall when selecting a gear. However, since most people don't bother to change their TC, then this thread is a pretty good way to determine gear ratios.


Also, I would like to add, there is NO advantage like mechanical advantage.
You are not being "that guy". In fact, I am glad you brought this up. I am assuming that a lock up stall is used such that when you are on the highway cruising, the converter is locked and there is no slipping.
 

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GMFS Enforcer
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bump
 

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Bump
 

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