GMC rear end gear change?

[Jlwright]

I need to change the rear end gears in my 2013 GMC Sierra from 3.42 to 3.73 gears. Does anyone know what all this involves? IN older trucks only the speedometer gear would need to be changed. I'm sure there is more to it on a late model. I hope the computer doesn't have to be changed and a bunch of other stuff that will make it not cost effective. My truck has the 6 speed auto transmission and 5.3 engine with transmission cooler and tow haul mode.

[Hercules1978]

Aside from the obvious gears, the computer will need to be reprogrammed for the gear ratio.

[Jlwright]

Is reprogramming a dealer only thing or can a good auto repair shop do this?

[bsmith0404]

If they have the scanner to reflash the computer any shop can do it. My experience is that a good transmission shop is more likely to have it.

[concours]

Hope that you have a 2WD if it's a 4X4 you have to change the front diff also

[Jlwright]

Oh crap, I knew that couldn't be that easy!

[notanlines]

I'm not certain if you'd even realize you have different gears. This is about an 8 percent change equaling about 150 rpm at 60 mph. I'm not saying there is no difference, but you are going to be well into four figures for this changeover getting very little in return.

[Jlwright]

Well I guess that is not an option. My truck only has 4000 miles on it but I wasn't planning on buying an RV when I bought it. Guess I'll see how it does.

[T3/T4 Hybrid]

If you change gears in your truck it will not effect the speedometer. GM like most auto manufactures now take the speed calibration off of one of the wheels in the area of the ABS ring. Only changing tire size will effect the speedometer.


If you want to see how gear ratio and tire sizes changes effect your engine RPM here is a page that should help you with that.

http://www.crawlpedia.com/rpm_gear_calculator.htm

[Jlwright]

Thanks. I'll take a look. According to that calculator about 200 RPM's at 60 mph. That is using a 31 inch diameter tire. Not sure how close mine is to that without measuring.

[T3/T4 Hybrid]

Here is one tire size calculator I like. You can put in your stock tire size and then you can check it with other sizes you might want to change too. It will also show how the different tire sizes will change the speedometer and rpms.

http://www.tacomaworld.com/forum/tirecalc.php

[slow]

Originally Posted by notanlines

I'm not certain if you'd even realize you have different gears. This is about an 8 percent change equaling about 150 rpm at 60 mph. I'm not saying there is no difference, but you are going to be well into four figures for this changeover getting very little in return.

To add to other's comments: Have you considered going to lower profile tires with a resulting smaller outside diameter to achieve the equivalent rear end gear ratio? That would rule out LT tires but a lower profile tire has less sidewall roll anyway.

But as others noted, unless you are climbing mountain passes, your current gearing will probably be all you need.

[Jlwright]

Now that I understand what is involved I will definitely try the current gearing and see how it goes. Right now I'm sitting here watching the freezing rain so it will be a while before any camping?

[sourdough]

I would not recommend changing the ring and pinion in the rear and omit the front. You might get away with it if all you ever did was creep around in 4 wheel for short distances. If you put it in 4 hi, or worse yet, if it is full time 4x4 you will have driveline problems pretty quickly. Your transfer case will find it pretty difficult to figure out how to make the front tires spin at 60mph and the rear at 65mph (numbers are just an illustration).

[Jlwright]

I would not attempt to do just the rear without doing both.

[GaryWT]

I guess, as others have noted, what are you trying to gain? The difference does not seem like much. Many tow with your gears. I towed for 3 years with 3.15 gears without issue. Went all over New England through the White Mountains etc., worked fine. Good luck and Happy Camping.

[Jlwright]

Pretty much cancelled the idea based on what I'm hearing.

[{tpc}]

I think you will be ok with the 3:42 gears. I think in the newer trucks, with the additional towing packages (which I still am unclear as to what exactly they upgrade) they come standard with that gear ratio. It was my understanding that this was done on purpose, and has something to do with the six speed transmission as well.

You probably are or were like me....trying to figure out what would be the best for towing, especially if encountering a grade, etc. Then you look around and see a lot of people saying you want the 3:73 gears for this...but in some cases, the earlier generation trucks like mine than have the 3:73 gears also are only 4 speed transmissions. So I think that plays a factor into it.

I know when researching I couldn't figure out why for the towing packages it seemed they brought that ratio down. It didn't make sense. But maybe there is something else they are doing in the rest of the transmission to compensate for that.

Either that or they thought people would complain about gas mileage while towing so they wanted to make sure it was better when not towing? lol..

[Jlwright]

I'm thinking you are are right about the six speed transmission and the higher gears. I have only pulled the camper home from the dealer which is about 35 miles with hills and not much flat ground. It seemed to do a lot of shifting which I'm not crazy about. it is just something I'm not used to but will adapt.

[Hercules1978]

My 2011 Silverado has the 6 - speed Trans and 3.42 gears and the towing package. We pull a '15 Cougar 31SQB (too much tongue weight for a 1500 no matter what the dealer says). I hit the tow/haul button and set the trans to M5 (still shifts automatically but locks out 6th) and set the cruise at 60-62. Rpm's are reasonable (2000-2500 iirc) and fuel mileage is respectable (I think around 9-10 mpg.