Mechanics

DIFFERENTIAL GEAR SWAP

1999 Dodge Ram • 79,000 miles

I have got a 1999 Dodge Ram 4x4 Sport and I recently installed a 5 inch suspension lift and 35 inch tires, I have been reading a few of forums that say with this setup it puts a lot of stress on the drive train and before I does harm to the transmission I am thinking of swapping out the factory 3: 55 gears and replacing them with 4: 56 which is what everybody say's you need to run this setup. What will I have to purchase to do this swap?
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Fabian83
August 15, 2012.



How do you think changing the gear ratio is going to change the damage the lift kit will cause? There are different ways of raising the body. Some put a lot of stress on rubber bushings, some do not. All greatly increase the drive shaft angles that were carefully designed in to reduce vibration and wear. To understand what happens when that is wrong, drive any '80s or '90s GM pickup or van.

You're going to have to change the gear ratio in the front differential to match the rear one unless you want the front and rear wheels to turn at different speeds in four-wheel-drive. Engine noise will go way up. Luckily your truck has a solid front axle. With independent suspension, or worse yet, Ford's twin I-beam front end, there is no way to eliminate tire wear with any alignment settings. If your lift kit included a dropped pitman arm to keep the same geometry for the steering linkage, handling should remain somewhat normal.

Caradiodoc
Aug 15, 2012.
Yes it came with a pitman arm and the steering is good, what I wants to know is can I install the 4: 56 gears by just purchasing the ring & pinion.

Tiny
Fabian83
Aug 15, 2012.
Yup, but you have to do both axles and they need to be set up by a specialist to adjust the proper preload and backlash. I did two axles with the service manuals handy. I don't ever want to do another one.

I'm still confused on what you were told about stress. The stress comes from the rubber control arm bushings being in a permanent twist from the suspension being lowered to raise the truck. The gear ratio has noting to do with that. 4.56s will give you quicker acceleration and usually lower fuel mileage but it won't change anything else. I can see where someone might think there would be less pressure on the universal joints but they can take that. The wear is also a factor of how far the needle bearings roll back and forth in the cups each drive shaft revolution. At the design height the cups are meant to turn back and forth a little so the needle bearings never stay in one spot and create dents in the rolling surfaces. Raising the truck increases the drive shaft angle which does not eliminate that rolling back and forth but it makes them roll further which I suppose could make them wear faster.

I think I'd talk with a few more people before you go to such an extreme measure of changing gear ratio without knowing what they're trying to solve. The impression I get is someone is trying to sell you an unneeded service.

Caradiodoc
Aug 15, 2012.
All the Dodge forums that I have been reading say's that running 35 inch tires with 3: 55 gear ratio will make the transmission work extremely hard with all the down shifting trying to turn the big tires so they are saying using the 4: 56 gear ratio will help take strain off the transmission. Will I need to purchase new bearings when changing the 3: 55 for 4: 56 gears?

Tiny
Fabian83
Aug 15, 2012.
The bearings should be the same. Sounds like the idea is for the faster-turning drive shaft to offset the need for the wheels to turn slower since the outer circumference will be bigger. You may run into an issue with the speedometer. As I recall, the speed sensor is still run by a plastic gear in the tail housing of the transmission. The service manual should have a chart that crosses the tire size and axle ratio to determine the correct number of teeth on that gear. Some car models have a pulse generator, then the tire size and axle ratio have to programmed in with a scanner. Those have a very limited number of choices and if you modify something enough and the needed choice isn't listed, you're out of luck.

In your case, a higher gear ratio and larger tire circumference might offset each other enough that the speedometer will be close. Look for one of those roadside speed indicators to check your speedometer. I've found them to be pretty accurate.

Caradiodoc
Aug 15, 2012.