Gears & Drive Shaft on a lifted GMC

gnet158 · March 25, 2003

I had my truck lifted three weeks ago and had a few questions. I've read through may post but have been unable to get a straight answer to my questions. Maybe I'm too much of a newbie to understand. I am not mechanically inclined.

My truck is a 2002 GMC Sierra 2WD running a 4.8L, gears are 3.43. My truck has a 6' RCD suspension lift, 3' Body lift (not sure of the maker) running 315x75x16 Pro Comp tires, I used the Hypertech Power Programmer to Power tune my engine for 87 octane, correct odometer & speedometer, raise the tire speed limit (as advised by Hypertech), & finally installed a air intake system.

My truck runs great with no hard shifting or anything noticeably different than when she was when she was stock (this is after the Hypertech & air intake install).

Here are my questions.

1. Will running the stock gears harm my transmission over time? I know people recommend going with bigger gears. Even the shop mentioned this but only for a performance gain (no mention of transmission harm).

2. Do I need to have anything done to my drive shaft because I have lifted my truck? Or did the lift kit come with something to take care of this?

Thanks for your help!

Dihappy · March 26, 2003

Chances are good that your tranny will fail over time. Im no tranny tech guy, so im just letting you know what others have experienced.

If you say you dont notice any difference, i gotta say thats hard to understand. I had 3.73's and when i put 315's on i felt a big difference. I couldnt even use OD on the highway!

After installing 4.56's i was amazed!

Shaners · March 26, 2003

I would agree that a rear gear swap to something in the 4.10 range would greatly improve your trucks power.

Your stock 3.42 gears aren't really enough to pull the extra rolling resistance and wind resistance that you have from the larger tires and lift.

If you don't tow or carry heavy loads, the extra wear in the tranny will probably be neglegible, but I would consider a rear gear swap sometime in the future. It would be relatively easy in the 2wd truck, and I think you would be amazed at the difference.

gnet158 · March 26, 2003

Thanks for the replys.

Dihappy,

I ment after the mods I can't tell the difference. As for driving my truck in OD I'm assuming that's the same as "D"? That's the only gear I drive my truck in. I have had no problems driving on the streets and freeways with the bigger tires. However I do remember calling the dealer to find out if they could correct my speedometer (before I found about Hypertech) and he mentioned that the tranny on my truck is auto scening.

Shaners,

So if I understand you correctly running bigger tires verus running stock tires & towing/carrying heavy loads will have virtually the same wear on my tranny? Well that's OK in my book. But I will look at getting bigger gears sometime next year. If I get bigger gears in the back what needs to be done to the front?

Thanks!

Shaners · March 26, 2003

If I get bigger gears in the back what needs to be done to the front?

Thanks!

In your original post, you say that your truck is 2wd. If that's the case then nothing needs to done to the front.

And yes, I would say that running the bigger tires and lift, would approximate the same wear on the tranny as running stock tires, and carrying or towing a load.

bigmack · March 26, 2003

here is my 2 cents....

Transmission : I agree there may be impacts to the transmission. The killer to transmissions is heat. If you cruising around the highway unloaded in OD, heat won't be a major issue. Although there will be an impact due to your bigger tires and stock gears, with the engine turning at lower RPMs there is less transmission fluid heading to the cooler be it an aux. cooler or just the cooler mounted in the radiator as the transmission pump is run off the input shaft of the transmission, lower RPM less fluid flow.

If you are running around town in stop and go traffic, the loss of mechaincal advantage in the rear end will tend to cause your torque converter to work harder. I am assuming you are making the truck accelerate at the same pace as you did before the tires and lift. The extra gas pedal you are putting in for the same acceleration is lost partially as as heat in your tranny, some goes out the exhaust pipe and some goes out the radiator, but it is the added heat in the tranny that is the concern. The loss of mechanical advanatge on the rear end looks just like a trailer to the powertrain of your truck.

To be absolutley sure, you can install a transmission temp gauge. There are other threads on the forum discussing how to install. This way you are sure you are not hurting anything.

Gears: The shop is right... the gears will help return your performance. You should have noticed a drop in your Highway RPMs from approx 2200 to about 1800 (give or take). What this means is the engine is now running lower in the power band at any given speed and less HP and Torque are available but the same amount of power is required to overcome resistance (air, rolling etc). You overcome that by again pushing the gas pedal closer to the floor, and the heat story begins again. If you are in OD though the torque converter heat issue is somewhat negated as it should be locking up. It almost looks like a 5 th gear as you accelerate you will see the shift into fourth then another 100 rpm ish drop as the converter locks up. As the other guy said he can't even get into OD, this is pretty normal for big tires, stock gears. If you're running in third gear, torque converter heat is again an issue but the higher RPM will force more tranny fluid to the cooler. Same reason it is recommended to tow in Drive not OD.

The other reason to tow in drive is to prevent overheating the engine, same principle as the transmission heat, the faster the engine RPM the more coolant runs through the radiator. All these things are impacted by running the engine in an RPM range that is different than the factory.

I would recommend installaling the tranny temp gauge so you can monitor the temps and look at gears when the budget allows. The gears will deal with the majority of issues and allow the truck to run in the RPM range it was designed for.

With a 2WD there should not no other concerns on the front if your lift kit was a quality kit. i.e included pieces to manage the alignment and steering geometry etc. You may want to upgrade your brakes (even just pads) as you are trying to halt a whole lot more spinning mass in the big tires. I'm sure you have noticed you need to push the pedal harder than before for the same braking performance (distance).

Driveshaft- again the quality kit should have provisions for driveline angle. either a spacer to drop the rear of the tranny or the carrier bearing on the drive shaft (if equipped). If your drive shaft is out of whack you would be feeling a pretty good vibration in seat of your pants right now.If you look underneath the parts from these kits are usually pretty easy to spot (colors different than everthing else or it's just plain not rusty yet).

Now that I've blown half my lunch hour writing this, lets see if other members agree with the "long" explanation.

G

Threejaws · March 26, 2003

I would go with at least 4.10's when you get the money. I wouldn't take the chance with the tranny going out. Gears would be cheaper than a new transmission and it will perform better.

Lifted 2003, 2500HD

gnet158 · March 26, 2003

Again thanks for the reply's. Bigmack thanks for the info. I guess until I get my gears redone the rule I should follow is driving my truck in "3" to prevent my tranny from overheating and allowing it to get more of the cooler tranny fluid from driving her in higher RPM's right?

Thanks everyone!