GMC Sierra Gearing Help Please

[superbuickguy]

4.10

I have a H3 Hummer Alpha. That is a H3 with the 5.3 motor and 4L60e transmission. Fully loaded I'm tipping the scales at 6000 lbs. I have 315/75 16 tires. I've done a 1.5" torsion bar lift (all that's needed to clear 35s with an H3)

4.10 is a good compromise between decent pulling power and the inevitable fuel economy you're going to lose.

Something to consider - your front differential - they are not terribly robust, thus you may need to do a full consideration about what you really want the rig to do before you spend money that you'll lose when you make the next upgrade. For example, since you have a half-ton, you can swap in a light-duty 3/4 ton pumpkin in the front, then either replace the rear with a non-float 14 bolt (6 lug) or go whole hog with a solid axle swap front (SAS) dana 44 or dana 60 and a full float 14 bolt rear.

It all depends on your ultimate goal and how much you're willing to spend... a plan helps so you don't spend twice (especially since 3/4 front differentials tend to come in my more off-road friendly gear ratios). You honestly cannot get a better starting-point for a platform then a LS motor and 4L60e transmission.... axles and everything else is pretty easy to swap when compared to making electronics speak nicely to each other..

 

[ZMagic97]

4.10

I have a H3 Hummer Alpha. That is a H3 with the 5.3 motor and 4L60e transmission. Fully loaded I'm tipping the scales at 6000 lbs. I have 315/75 16 tires. I've done a 1.5" torsion bar lift (all that's needed to clear 35s with an H3)

4.10 is a good compromise between decent pulling power and the inevitable fuel economy you're going to lose.

Something to consider - your front differential - they are not terribly robust, thus you may need to do a full consideration about what you really want the rig to do before you spend money that you'll lose when you make the next upgrade. For example, since you have a half-ton, you can swap in a light-duty 3/4 ton pumpkin in the front, then either replace the rear with a non-float 14 bolt (6 lug) or go whole hog with a solid axle swap front (SAS) dana 44 or dana 60 and a full float 14 bolt rear.

It all depends on your ultimate goal and how much you're willing to spend... a plan helps so you don't spend twice (especially since 3/4 front differentials tend to come in my more off-road friendly gear ratios). You honestly cannot get a better starting-point for a platform then a LS motor and 4L60e transmission.... axles and everything else is pretty easy to swap when compared to making electronics speak nicely to each other..

Thanks for such an informative reply. I keep hearing that 4.10s from 3.73s won't do a lot, but it seems like you have me thinking otherwise.

I honestly plan on leaving everything else alone, only because my wife uses this truck for the kids so I can;t have it sitting too long, and I really don;t plan on doing anything too extreme: we use it mostly for forest roads, some more difficult trails, camping, and long highway drives.

 

[drewactual]

a fella above me said "don't fear the gear", and he's right..

couple of points:

the EPA sucks... they make this hard... they don't go out and drive every vehicle and determine economy, and the OEM reports what is mathematically determined by four factors in their little formula... those points are based off of a small percentage off stioch air to fuel ratio's, displacement of the engine, RPM's, and overall gear ratio (as measured after the tire)...

if you give a nerd those four factors, they can determine the expected economy of that truck... this is what those jerks do, but they ignore important factors that anyone who actually uses the truck figures out in due time.

let's suppose your rig is an automatic transmission equipped rig... this adds a bit of complication, so I'll use it..

your engine is balanced for air and fuel, and speed (RPM) out of the factory to produce the best power for what it's connected to... it's connected to a transmission, but first a torque converter... the TC is a fluid coupler- which is to say the engines crank spins a finned plate which disturbs the fluid causing it to swirl which causes another finned plate which is attached to your transmission input to spin... how it's designed determines how fast the input shaft responds- people speak of 'stall speed', and what they most often mean is at what speed the engines crank spins the plate connected to it before the plate connected to the trans input shaft overcomes the vehicles brakes ability to hold the truck still... at that point, the engine either stalls out or your tires lose traction.

how easily those tires break traction is gong to be based off of what kind of leverage the engine has on the tires. that leverage is calculated starting with the engine and where the majority of power is generated, which is based on the camshaft profile, the available air, the catalyst added (fuel), the displacement and square of the bore (length of piston travel or stroke and diameter of bore), its volumetric efficiency, and how fast it spins up...

then... leverage and resistance...

the leverage that engine and that profile has on the transmission- via that TC, which is the first place the engine applies leverage...
after the TC, the transmission adds leverage through whatever number of gears it has.
after the transmission the drive shaft is used to torque the axle gear...
the axle gear torques the wheel/tire through whatever reduction it's meant to.
the size of tires (height mostly if not primarily, but don't forget weight as spun weight is x4)

can you see how this game is tricky?

we buy rigs that meet our needs or fancy the closest, not the best... to make it 'best' you gotta dig into it and make it do what you want it to... but to change axle gears alone (or tire size for that matter)? you're playing with fire, as something else is going to be pushed to its limits, or worse by my reckoning, NOT being used to it's advantage.

So.. INSPECT your rig right now... figure out where you're not taking best advantage of what you have... hopefully I've explained how the whole machine works in concert, now, and figure hard before you just attack it at it's easiest (maybe not best) solution...

I've re-geared no less than three times on rigs over the years... the first time as an easy solution... what I found is why I'm writing this friggin' novel.. the first thing I found was the engine worked a LOT less at ALL speeds as witnessed by a vacuum gauge. High signal is GOOD... I'll lay down a bet right now that a 350cid CSB MPI will get equal or better economy @ 3k RPM and -14 inches of vacuum as the same motor turning only 1600RPM but holding only -8~9 inches of vacuum... the load on the engine is a lot less with the higher vacuum, and it means it's turning along within it's power range... if it's in it's power range (dictated by the cam/VE/square/and TC) you have power only a slight amount of pressure on the skinny pedal as opposed to having to stand on it and drop a gear (increase leverage). I've realized that more leverage with a lower stall rate of the TC (and a TC w/ lock, which makes it 1:1 with crank spin) usually equals not only better economy but better use of power...

I say inspect your rig right now to see if your TC could be swapped to achieve a similar end... of your cam could be swapped for moving the power range to an RPM more suitable for what you need. Understand, though, that just altering the gears in the axle may upset the balance your rig was engineered for.

ALL THAT (and it was a bunch) SAID- IF you are altering axle gearing for the express purpose of offsetting the use of taller (or substantially heavier) tires, just use the calculator as somebody else said and choose the one that gets as close to stock OVERALL ratio as you can.. Overall ratio is a combo of all the leverage and translated to how far your wheels spin for each revolution of the drive shaft.

it all works in concert... and this novella of mine I hope cuts through why selecting the proper ratio is important, as it will impact things you may not have considered.

 

[ZMagic97]

drewactual,

Thank you very much for such an informative and detailed response. After reading your post and realizing how much tune and truck's actual RPMs are vs the charts I'm feeling much more educated on making a decision that is best for my needs. I'll be taking some more time to insect my truck and move forward with a decision. Thank you again.

 

[justcuz]

4.10

I have a H3 Hummer Alpha. That is a H3 with the 5.3 motor and 4L60e transmission. Fully loaded I'm tipping the scales at 6000 lbs. I have 315/75 16 tires. I've done a 1.5" torsion bar lift (all that's needed to clear 35s with an H3)

4.10 is a good compromise between decent pulling power and the inevitable fuel economy you're going to lose.

Something to consider - your front differential - they are not terribly robust, thus you may need to do a full consideration about what you really want the rig to do before you spend money that you'll lose when you make the next upgrade. For example, since you have a half-ton, you can swap in a light-duty 3/4 ton pumpkin in the front, then either replace the rear with a non-float 14 bolt (6 lug) or go whole hog with a solid axle swap front (SAS) dana 44 or dana 60 and a full float 14 bolt rear.

It all depends on your ultimate goal and how much you're willing to spend... a plan helps so you don't spend twice (especially since 3/4 front differentials tend to come in my more off-road friendly gear ratios). You honestly cannot get a better starting-point for a platform then a LS motor and 4L60e transmission.... axles and everything else is pretty easy to swap when compared to making electronics speak nicely to each other..

4.10's are just a 10% increase in gear. OP there are tire diameter to recommended gear charts on the Internet. They give 3 choices of gears the the optimum one being in the middle. I am not so good at linking but if you search "tire diameter to gear ratio" you should find it. Randy's Ring and Pinion may have it on their site also.
Pulling the axles at the wrecking yard is the budget way to do this but you need to carefully inspect the gears before making the purchase.
Superbuickguy, I quoted your post for 2 reasons, 1. What size front pumpkin is in your H3 and is it cast iron? 2. To the best of my knowledge the light duty 3/4 front pumpkin or clamshell that holds the gears is the same as a half ton and is aluminum. At least it is on my 1993 light duty 6 lug K2500. Looking on the Internet all I see is 2 ring gear sizes for front axles and 3 types of housings.
One aluminum for all 1/2 ton and light duty 3/4 ton 8.25 ring gear and two for the 3/4 and 1 ton with the 9.25 ring gear one aluminum and one cast iron. Do you have any knowledge of anything different than this info?

 

[justcuz]

Drewactual, let me add a little to your torque converter explanation. In actuality the brakes should hold the vehicle still even if the throttle is floored.

Stall speed is actually when the vehicle on level ground without any brake application will move forward with throttle application.
The rpm that the vehicle starts moving forward (the fluid coupling first transmits power to the transmission) is your stall speed.
A torque converter is a liquid clutch. When you step on the gas the liquid clutch engages at a preset rpm. That's your stall speed.
I just said the same thing 3 different ways, so everyone can pick an explanation that suits them!:ylsmoke:

 

[drewactual]

heheheehheeee- as complicated as I made that post of mine, I actually chose not to discuss the different types of stall, because it gets confusing quick... the easiest way to explain it by my reckoning is the way I did- but you're on it, too... what i refer to as the explanation you used 'when the car starts to move' as the engage stall.. there is also 'flare' stall, which is the free spin gap between gears which gives the lower gear a ********** to get going, usually around 300rpm or so, and what shift kits try to narrow with pressures... there are at least two more that are valuable, but, hell, I can't recall it all without a refresher!!!

oh.. flash stall, the rpm bump as you take you're foot off the brake and when it reacts- like a little 50rpm bump or so... that's another one...

 

[drewactual]

and since this forum engine won't let me edit above, I'll just post a new one...

if the brakes are firmly planted and the gas floored while in gear, the engine will stall out. it won't happen instantly, but it will once that TQ locks firmly to the input- which is what i should have said instead of suggesting the tires would break free... it would be wicked cool if the tires broke free- and on a really powerful engine or a vehicle with brake issues, or both, it would break free. i wouldn't recommend trying this...

 

[Mickey Bitsko]

I had 3.73 went to 4.56 's went up 1" on tire height and got my top end/rpm/cruising speed with no mpg loss.

 

[TwinStick]

My $.02,

Do not fear the gear. IMHO, it is not worth the time, effort or money to go up one gear size.

 

[justcuz]

drewactual, your going to make me look all that stuff up! I was trying to simplify it a bit. Lots of good info in your post. I know that due to the broader torque range of the Chevy LS engines they get better mileage at higher RPMs than they do in 4th gear at legal speed limits.
I've had this discussion with a couple friends and we are trying to estimate the highest torque multiplication at low speed a converter can achieve. I believe 2-1 is possible, so first gear in a 4L60E, 6-1 at low RPM's (just above stall).
We were trying to calculate low range crawl ratios. So my Suburban with a 4L60E in low gear, low range with 3.73's would be 6x2.72x3.73=low range crawl ratio. What do you think?

 

[drewactual]

drewactual, your going to make me look all that stuff up! I was trying to simplify it a bit. Lots of good info in your post. I know that due to the broader torque range of the Chevy LS engines they get better mileage at higher RPMs than they do in 4th gear at legal speed limits.
I've had this discussion with a couple friends and we are trying to estimate the highest torque multiplication at low speed a converter can achieve. I believe 2-1 is possible, so first gear in a 4L60E, 6-1 at low RPM's (just above stall).
We were trying to calculate low range crawl ratios. So my Suburban with a 4L60E in low gear, low range with 3.73's would be 6x2.72x3.73=low range crawl ratio. What do you think?

yup- a man said 'don't fear the gear', and it could be translated 'don't fear the RPM' just as easy... it's all about engine load... many makers lug the engine in o/d- which is okay, I guess, as the engine is producing enough power to maintain, but it's harder on the engine than turning several hundred RPM more and sitting in it's power curve. The mileage (economy) may be about the same, but that's only in a perfect world where terrain doesn't alter and winds don't blow. When an engine sitting low down in the RPM's in a high gear encounters greater resistance, it has to drop a gear or it has to jump upwards of 500+ RPM to overcome, which over a haul of even a hundred miles, will cost more fuel than the engine running higher RPM's, not lugging, and in the power band- the differences being the power required to overcome whatever obstacle is right there- 50, 100, 150 RPMs above the 'cruise' band... and the other difference being the load on the engine as measured by a vacuum gauge... the signal is stronger, meaning the engine is comfortable, and meaning if you have a MPI PCM/ECU controlled ignition and fuel trim, the ignition is advancing to it's safe ceiling and the STFT/LTFT are steady, which both equate to better management of power and evaluated by it's greater economy...

about the crawl: I'm out of my element in that discussion with a certainty- but i will offer that you may be able to achieve silly application of TQ at low wheel RPM's by choosing the right combo of critters in between, but one of those critters better be a plate and fin cooler for the ATF and at least a 3kCFM electric fan to cool it...

 

[team140]

Our trucks are nearly twins!

For what it's worth, according to the build sheet my truck came with 3.54 gears, but with the 35s the speedometer reads 5MPH fast. Back of the envelope math says it actually has 4.10 gears in it now. I just picked it up a couple of months ago, so don't know the entire history of the truck. The topper was installed yesterday.

The 4.10s with the 5.3 and 35s is a GREAT combo. I just made a trip from Gainesville, FL to Nashville, TN and back and the truck never really hunted for gears in the hills of Chattanooga, TN.

 

[ZMagic97]

Thanks for the additional info everyone.

And team140, sweet rig! I really like the green.

 

[justcuz]

Here is the gearing chart I was referring to. Keep in mind the RPM's on the chart are for 1-1 final drive, reduce that by .30 for your overdrive transmission.