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GEARS!!, Please read before posting gear ?'s. Updated 11/11/03

Here are some really good equations for determining what gear ratio you may want to concider.
Remember that this is reference only and is based on equasitions from simple math.
Your tire size may be listed as 33/35/37, etc. but the actual measuremnet of the tire could differ once it is mounted on your rim.

Also this write is assuming you have an E4OD, or similar auto tranni.
These are figures that I came up with, to do your own just do:
Circumference of tire divided by the gear ratio you want.
Then compare you answer to the stock forward measurement of 25.5"s and then you will know if it fall over or under.

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Stock 29" tire (235/75R15), with 3.55 gears, 90.7" is circumference of tire, thus for every turn of the drive shaft, you go forward 25.5" or so.

33" tire, 3.55 gears = 103.7" circ, forward 29.2", about 15% more than stock
33" tire, 4.10 gears = forward 25.3" basically same as stock
33" tire, 4.56 gears = forward 22.7", about 11% less than stock

35"s with 4.10s = 26.8, 5% over stock roughly.
35" tire, 4.56 gears, 110.0" circ, forward 24.1", about 5% less

37" tire, 4.56 gears, 116.2" circ, forward 25.5", identical to stock (within error)
37" tire, 4.88 gears, forward 23.8", about 9% less than stock.

38.5" tire, 4.56 gears, 121.0" circ, forward 26.5", about 4% more than stock
38.5" tire, 4.88 gears, 121.0" circ, forward 24.8", about 3% less than stock.

FYI, for those who are thinking they can run a 35" tire with 3.55 gears...

35" tire, 3.55 gears, 110.0" circ, forward 31", about 22% more than stock. That means that your motor is working (and your clutch or torque converter) quite a bit harder to move your truck. That's why it dogs so bad.

If it falls "under" your motor does not have to work as hard to turn the tires, but it does it at a higher RPM. This means that as you go more "under" you lose top end. If it's "over" you'll have a higher top end, but less grunt.
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Thank you.
 

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gobucs posted a calc...try it out. and you have a c6, no overdrive, so your rpm is going to be even higher.

3.73 is pretty much between 3.55 and 4.10, so just estimate.
 

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I would say you need 5:13's for a set of 44's and a c6 tranny also with a np 203 or np 205 transfer case. That would put your RPM's at about 2690 @ 65 MPH.
 

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Discussion Starter #9

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still confused

Ok, im still confused. If i run my 35's with stock gears, its gonna give me less power and torque? and if i go to 4.1 its better?? :shrug
 

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Gear swaps require a press, setup bearings if you do not have a high priced bearing puller, (clam shell type), a micrometer, a dial indicator, and a copy of the Yukon Gear installation manual. (Randy's R&P has it as a .pdf on their web site). Tolerances are measured in .000 inches. As well as a box full of hand tools and then shop air to torgue the pinion down.


As far as working the engine with 35's and stock gears....it's all about the power band. Your truck looks to do about all it can do before 2700 rpm. It's a truck motor anyways. Your truck engine willl have to increase power to turn the bigger tires. Being a '96, you'd be better off doing the around town driving with the OD off. Your really working the transmission more thean the engine with this setup. Although, a tired engine will also start to show problems as well. Oil pressure loss, overheating, etc.


It's not a BIG issue on the road. Heck, if 90% of your driving is pavement, you'll probably never have a serious problem. Off road obstacles are going to bite you though. Taller tires and stock gears will make the drivetrain work that much harder. You'll be overheating from the start.


Bob K.
 

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You could also add VIN codes for axles to this as well. eg. under "axle" on your door's VIN sticker "H9" is 3.55's with a limited slip, "C5" would be 4.10 with a limited slip. This could help when searching the local wreckers.
 

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Beerman said:
Also this write is assuming you have an E4OD, or similar auto tranni.
So how off would this figures be for those running a ZF 5 speed?

James
 

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Discussion Starter #14
Sorry, for a zf 5 speed you will hve to figure out what your rpm band range is now, then do the math listed for each gear. once you have that then you will know your overall rpm range for 5th gear
 

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Beerman said:
.....
Circumference of tire divided by the gear ratio you want.
....
33" tire, 3.55 gears = 103.7" circ, forward 29.2", about 15% more than stock
Now my 8th grade math class taught me that circumference was 2*pi*r. So...

EDIT: Removed my strange math.

EDIT: Ha ha. Except that tires sizes are diameters and I was forgettig the 1/2 first that made it SEEM like the 2 was missing from the equation. :histerica
 

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but, once you get larger tires moving does it require any more power to keep them moving than small tires? or is it just about the same difference?
 

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hmmm, I've found that stock brakes work pretty decently with 33's. I don't ever really put myself in situations that require really quick stopping though.
 

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Hi all

I am Lifting my 1991 FSB and and installing 35 inch tires.
Not using it for off road; the only time I will engage the front is in stuck situations.
So for those of you who have successfuly done this the gear setup
what is the ideal gear ratios for the front axle and rear axle.

Currently the front is 3.54 and don't know what the rear is

thanks
 
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