Gear Replacement - Which Ratio

[RichardSenn]

I need a little help from the experts. I have an 06 Silverado CC 5.3L 4x4 that I have decided to keep and use primarily as an offroad vehicle and hunting truck. I will be replacing the stock 265/70/17 tires for 35x12.50 tires so I want to regear both the front and rear so the trans doesn't hate me. Problem is I don't know which ratio to go with nor do I know which brand of gears are quality gears. If you ask on the Chevy forums and most 4x4 shops it's usually some 16 year old kid giving the recommendations so you can see my frustration.

I will still be driving the truck on the highway since my hunting lease is 120 miles round trip and I go every weekend from Oct-Jan. Because of that I'd like to stay as close as possible to stock RPMs while maintaining plenty of power. Online calculators tell me to go anywhere from 4.10's to 4.56 gears.

What are your recommendations?

 

[Rand]

what are the stock gears 3.73?

you are only going up about 10% in tire size I'd try it without spending the 1000$. You have a torque converter for starting out.. and can always lock out overdrive if it likes to switch gears too much.

4x4 low will more than make up for 10% bigger tires offroad.


just my opinion but I'd try it first.

 

[supton]

http://www.gordon-glasgow.org/tirecalc.html
There are better calculators, but this is the one I use.

265/70/17 is 638 rev's mile. Oops, that one doesn't do the 35's, so let's try here:
http://tire-size-conversion.com/tire-size-calculator/

35x12.5x17" is 577 rev / mile. About 11% difference? I think you have 3.42's; if you do, especially with the 4spd automatic, I'd probably go to at least 4.10's, but that's on the assumption that it's too-tall geared, which many find 3.42's to be with the 4spd auto. Also: bigger tires sap more power, so if they are wider, or simply heavier, you might want to go down another gear ratio anyhow.

Anyhow, for this tire change, if you had 3.42's you'd want to go to 3.73's for the same rpm; and if you had 3.73's you'd want to go to 4.10's. [Ex: 638/577*current_gear gives equivalent new gear required.]

As for what ratio you have right now: look in the glovebox, and look at the codes for one of the following:
G80: locking diff
G76: posi diff
GU4: 3.08
GU5: 3.23
GU6: 3.42
GT4: 3.73
GT5: 4.10

I think I'm with Rand: it won't hurt to at least try with stock gearing. If you absolutely hate it, then you might want to go more deep than you initially are guessing right now (that is my SWAG).

Edit: if you do have the 4spd auto, I would pay attention and try to not drive with the convertor unlocked. If it decides to drive on the highway "often" with it unlocked, well that's not a great thing.

 

[Ramblejam]

As aforementioned, we need to know what your current gear ratio is...

Do you already have a lift on it?

 

[JZiggy]

As others have said, that's a small enough tire size difference (10%) you may be OK with stock gears. It's a bigger issue in the Toyota offroad world where many people have
Larger tires also give you greater rotating inertia, rolling resistance, and wind resistance. If you do regear I would err on the side of gearing lower (ie, higher RPMs on the highway) so you have more available torque to balance these factors.

If you regear it may also be a good time to install a locker of some sort. That makes a huge difference offroad.

 

[bullwinkle]

If you're not towing, the 3.73s are a good choice; if you're towing, the 4.10s would be better. This is the reverse of the dilemma I had with the F-450-the factory 5.13-1 ratio was so high that 65 on the highway was drinking diesel & the 7.3 was screaming. I went with 3.73s, 4.10s would have probably been better for the weight & occasional light towing. If you have the 4L60E you want to go lower to lessen the strain on the overdrive gear, they are weak from the factory.

 

[meep]

once you put tires that large on, you'll sort of be limited to your top speed some degree by noise/comfort/handling anyway. You won't be doing 80 on 35s, and won't be subjected to engine rpm at 80 that the truck used to be able to do. (or whatever number).

a 10% increase, you will feel, no question. But as others have said, it may be tolerable. Tires first.

I think supton probably nailed it above. not much to add-- his numbers look good to me.

I have lifted and put larger tires on a couple of vehicles and I can say with experience that being undergeared is no fun. If this is a hobby and you want to regear yourself, I think it's great. If you are going to pay a shop, it may not be worth it.

My other thought is if you are going to regear, then consider the extra step. i.e, don't go from 3.42 to 3.77. go one more to 4.10--- as you will be fighting more than just tire diameter. But that's subject to taste, and if you might go back smaller one day. THAT is a personal judgment call--- and you may not know until you try it stock. My truck is over-geared by its factory tow package, and I enjoy having the extra scat. I probably pay a mpg penalty, but the vehcile doesn't feel anemic.

 

[95busa]

4.56-1 for the win

 

[cchase]

We had 4.10 ratio on our 2500HD. It was fine on the highway with stock wheels/tires.

Don't spend too much time fretting the jump from 3.73 to 4.10. You won't go to 4.10 and (especially with larger tires) feel some huge difference on the highway (ie suddenly terrible gas mileage) and regret the change.

 

[RichardSenn]

Sorry for the late response, work got crazy today and we finally put the kids down after soccer practice.

I should have given more details in the original post. The truck currently has 3.42 but also keep in mind there were two stock tire sizes in this model with 245/70/17 being the smaller size so I'm not sure if the 3.42 gears were really meant for the 245's for the 265's.

I will occasionally be towing a 5600lb travel trailer back and forth to the lease. Actually, I'd love to pull it with the truck vs. the 06 Tahoe we have but that won't really be practical since lifting and increasing tire size are already going to cost me some mpgs.

The truck currently has a 4" suspension lift and 285/70/17 tires so I'll be changing the lift to a 6" lift to clear the 35x12.5 tires. The highway to and from the lease is a turnpike with a speed limit of 75 mph which I typically run 80 mph to keep up with the flow of traffic. I'm trying to find the quietest gears and tires which in 35x12.5 really isn't going to happen.

Most calculators I run say 4.10 gears, but like I said all the recommendations are 4.56. I believe there is one gear manufacturer that makes a 4.27 so maybe that would be a happy medium.

I will definitely see how the truck does before regearing but I'm making the offroad changes with the plan to regear and rebuild the transmission since it has 186K on it currently.

 

[Ramblejam]

So, 35's are only a couple of inches larger (usually around 34.8" diameter) -- is all the cost of regearing really worth 170 RPM's?

You're wanting to put on an expensive lift & gears, but want nice and quiet tires on this self-described off-road/hunting truck? Please, don't be that guy.

Unless this is just a **** measuring contest with hunting buddies (who has the tallest, baddest-looking truck), tires matched to the terrain (of the same size you have now), traction devices in the axles, and proper recovery equipment will go much further in having an effective, usable vehicle -- and, you'll be able to tow the trailer!

 

[RichardSenn]

Originally Posted By: Ramblejam
So, you should currently be running right around 2,800 RPM's @ 80. 35's are only a couple of inches larger (usually around 34.8" diameter) -- is all the cost of regearing really worth 170 RPM's?

You're wanting to put on an expensive lift & gears, but want nice and quiet tires on this self-described off-road/hunting truck? Please, don't be that guy.

Unless this is just a **** measuring contest with hunting buddies (who has the tallest, baddest-looking truck), tires matched to the terrain (of the same size you have now), traction devices in the axles, and proper recovery equipment will go much further in having an effective, usable vehicle -- and, you'll be able to tow the trailer!


Trust me I know there will be added noise with 35'' tires, especially if they are MTs. That said there are some MT tires that are definitely quieter than others. The same goes with gears. I know I'm not driving a Lincoln town car and the noise won't bother me since I know it's expected but if I can reduce some of the noise by finding the right tire/gear then I might as well do that.

At 80 mph I'm not running anywhere near 2800 RPM, it's more like 2000. Think I was running around 1825 RPM tonight at 70 or 75mph can't remember which.

It's definitely not an contest between buddies, but I can tell you I do want the off road look. Why, well because I've never done it due to the fact it wasn't practical as my daily driver, I was to young and couldn't afford it, etc.. and being that this truck is paid for and I have another vehicle to drive I've decided to keep it forever and make it look and drive the way I want and do things right. If it was a contest I wouldn't be worried about gears, I'd be worried about getting the biggest possible tires and biggest lift possible at the lowest price with no regard to how the truck would perform. Essentially, I'd be like the 16 year old kid on all the chevy forums who has a great looking truck but is replacing ball joints, transmissions, and front end parts every several thousand miles because he didn't take into consideration anything else but beating his buddies.

 

[Ramblejam]

Originally Posted By: RichardSenn
At 80 mph I'm not running anywhere near 2800 RPM, it's more like 2000. Think I was running around 1825 RPM tonight at 70 or 75mph can't remember which.


That was at 1:1 ratio, not overdrive.

Originally Posted By: RichardSenn
Essentially, I'd be like the 16 year old kid on all the chevy forums who has a great looking truck but is replacing ball joints, transmissions, and front end parts every several thousand miles because he didn't take into consideration anything else but beating his buddies.


So, you have more money than the 16 year old. Great. The intentions are identical. Good luck with whatever you decide.

 

[RichardSenn]

Originally Posted By: Ramblejam
Originally Posted By: RichardSenn
At 80 mph I'm not running anywhere near 2800 RPM, it's more like 2000. Think I was running around 1825 RPM tonight at 70 or 75mph can't remember which.


That was at 1:1 ratio, not overdrive.


Ahh, well in that case I probably am at 2800+ RPM when I'm not in overdrive.

 

[Jim Allen]

Use the Equivalent Ratio formula. IMO, 35s on 3.42:1 ratios will result in very sluggish acceleration, needlessly high trans oil temps and a significant drop in city mpg.

Html:
Equivalent Ratio Formula-



New Tire Diameter

_______________ X Original Gear Ratio = new ratio

Old Tire Diameter



Example:



A Chevy half ton truck owner with 265/75R-16 tires (about 31 inches) with 3.43:1 gears wants to run 35s.



35

__ X 3.42 = 4.86

31




The nearest available ring & pinion ratios are 3.73 or 4.10, so unless the truck is particularly powerful or the owner wants to opt for highway fuel economy vs acceleration and trail performance, use the 4.10s. The city mpg will be better with the 4.10s by a tad. The 3.73s will approximate the original tire size/axle ratio relationship and the speedo will probably read pretty close. The bigger tires in and of themselves (weight, rolling resistance, etc.) will cause a drop in performance and a loss of aerodynamics at higher speeds. For that reason, IMO, the 4.10s are the best overall choice to approximate the stock performance but 4.56:1 is the best performance choice. In general, people with the common pickup truck power-to-weight ratios are most happy with performance using the following tire dia./axle ratio combos:

31s=3.73
33s=4.10
35/36=4.56-4.88
37/38=4.88-5.13

 

[RichardSenn]

Originally Posted By: Jim Allen
Use the Equivalent Ratio formula. IMO, 35s on 3.42:1 ratios will result in very sluggish acceleration, needlessly high trans oil temps and a significant drop in city mpg.

Html:
Equivalent Ratio Formula-



New Tire Diameter

_______________ X Original Gear Ratio = new ratio

Old Tire Diameter



Example:



A Chevy half ton truck owner with 265/75R-16 tires (about 31 inches) with 3.43:1 gears wants to run 35s.



35

__ X 3.42 = 4.86

31




The nearest available ring & pinion ratios are 3.73 or 4.10, so unless the truck is particularly powerful or the owner wants to opt for highway fuel economy vs acceleration and trail performance, use the 4.10s. The city mpg will be better with the 4.10s by a tad. The 3.73s will approximate the original tire size/axle ratio relationship and the speedo will probably read pretty close. The bigger tires in and of themselves (weight, rolling resistance, etc.) will cause a drop in performance and a loss of aerodynamics at higher speeds. For that reason, IMO, the 4.10s are the best overall choice to approximate the stock performance but 4.56:1 is the best performance choice. In general, people with the common pickup truck power-to-weight ratios are most happy with performance using the following tire dia./axle ratio combos:

31s=3.73
33s=4.10
35/36=4.56-4.88
37/38=4.88-5.13


So for my setup I'd be at 34.8/31.9 x 3.42 = 3.73 Assuming everything was the same I would go with 3.73's but its not with the added weight, rolling resistance, etc so I should step up another gear to 4.10's.

The truck has plenty of power currently and that's with the larger 285 D rated tires I have on currently. Let's say the 4.10's get me back to the performance I have now, how much would going a step further to 4.56's increase my towing capacity?

 

[sw99]

4.10 will work and give you the best MPG's. Personally I would do 4.56's, but I like a bit more power off-road capability (I have 5.38's and 37's). As for gears, USA Standard Gear is hard to beat IMOP.

 

[RichardSenn]

Originally Posted By: sw99
4.10 will work and give you the best MPG's. Personally I would do 4.56's, but I like a bit more power off-road capability (I have 5.38's and 37's). As for gears, USA Standard Gear is hard to beat IMOP.


I've been thinking of going with Standard or Yukon gears, those have been the two most recommended.

The more I think about it the more I think the 4.30 gears might be perfect since I'm on the fence of 4.10's or 4.56's.

 

[meep]

I would go towards the 4.56 as well. your intentions are not really in line with highway cruiser and mpg. if you are lifting to a 6", plus the larger tires, you want the extra gear. If you are towing, you want it even more.

BUT--- pulling a 5600lb trailer with a highly lifted and large-M/T tired truck doesn't make sense to me. That seems to sacrifice safety for looks/hobby. 5600lb is not a small load, and the typical lift kits of spring blocks/longer shackles can compromise the stability of suspension /parts/ in addition to the general vehicle dynamics. I would not go over a 4" and 33's. a MT is not a good hwy tow tire, and longer sidewalls is not going to help keep you planted in crosswinds and defensive maneuvers while the 3-ton brick behind you amplifies every motion. I'd put the money into armor and recovery, trans rebuild/driveline and go quality on the 4" and 33's. AFter that I'd pull the motor and have some serious fun with that--- build it, stroke it, tune for torque and growl (not an rpm-screamer).

do be safe while you have fun!

 

[RichardSenn]

Originally Posted By: meep
I would go towards the 4.56 as well. your intentions are not really in line with highway cruiser and mpg. if you are lifting to a 6", plus the larger tires, you want the extra gear. If you are towing, you want it even more.

BUT--- pulling a 5600lb trailer with a highly lifted and large-M/T tired truck doesn't make sense to me. That seems to sacrifice safety for looks/hobby. 5600lb is not a small load, and the typical lift kits of spring blocks/longer shackles can compromise the stability of suspension /parts/ in addition to the general vehicle dynamics. I would not go over a 4" and 33's. a MT is not a good hwy tow tire, and longer sidewalls is not going to help keep you planted in crosswinds and defensive maneuvers while the 3-ton brick behind you amplifies every motion. I'd put the money into armor and recovery, trans rebuild/driveline and go quality on the 4" and 33's. AFter that I'd pull the motor and have some serious fun with that--- build it, stroke it, tune for torque and growl (not an rpm-screamer).

do be safe while you have fun!


Towing with the truck would be nice for the added bed capacity but I do plan on towing with the Tahoe. I'll just use the truck to pull the travel trailer to the lease to start the hunting season in October and back to the house in January.

MPG is only a concern because I do drive to and from the lease several times a week during the hunting season. I don't want to be sucking a tank of gas every two trips. If MPG really drops off I can always leave the truck out there at the lease during hunting season.