Rear end ratio

[FCDGMC]

I have a 94 K2500 with the 454 engine. I have determined from another member's response that this vehicle's GT-4 rear end code equates to a 3.73 ratio. I do not use this vehicle for towing, and probably will not any time soon. I would like to put a higher (lower ratio) rear end gearset to increase the fuel economy. I have several questions:

1. How low a ratio is practical and/or will make a significant improvement without losing too much power (realizing of course that this engine has ample low end power)?

2. Can I assume that to do the job right I would have to change the front end gearing as well so that when the vehicle is in 4wd the two axles aren't grinding against one another?

3. Which parts typically have to be replaced in such an operation-is it just the ring and pinion gears?

4. Lastly, are there any other considerations when performing a modification like this?

Any guidance is greatly appreciated!

FD

[maxwedge]

Both diffs have to be done, if you have never done one before, and don't have the proper tools I suggest a pro do this, your truck may not be the proper environment as a learning tool. 3.42 is probably the next size lower, which would allow the engine to turn about 10% slower for a given speed. I think the fuel mileage savings would take a long time to make up for the cost of this project.

[FCDGMC]

I certainly appreciate the input. You're not the first person I've heard say it's something best left to a pro. Can you tell me what tends to make this procedure difficult? I have done some types of repair; water pump, valve covers, transfer case seal, brakes, struts, all types of fluid changes, and even pulled one of the rear axles off this vehicle to reseal; but never anything quite this involved. I consider myself pretty mechanically inclined.

Any feel for what it would cost to have a pro do it?

You mentioned a 3.42 ratio, is there any reason you wouldn't continue to say a 3.23?

Thanks again.

FD

[FCDGMC]

How low (in say a 3-series gear set) is reasonable without sacrificing too much torque (keeping in mind I won't be using it for towing)?

[Elbert]

Originally Posted by FCDGMC

I have a 94 K2500 with the 454 engine. I have determined from another member's response that this vehicle's GT-4 rear end code equates to a 3.73 ratio. I do not use this vehicle for towing, and probably will not any time soon. I would like to put a higher (lower ratio) rear end gearset to increase the fuel economy. I have several questions:

1. How low a ratio is practical and/or will make a significant improvement without losing too much power (realizing of course that this engine has ample low end power)?

2. Can I assume that to do the job right I would have to change the front end gearing as well so that when the vehicle is in 4wd the two axles aren't grinding against one another?

3. Which parts typically have to be replaced in such an operation-is it just the ring and pinion gears?

4. Lastly, are there any other considerations when performing a modification like this?

Any guidance is greatly appreciated!

FD

Personally like others have said I would not change the ring gears, and if you do you'll have to change both the front and rear. In my view the only practical gear to go any higher to would be the 3.42. I would not go lower than a 3.42. You will also have some speedometer error issues.

This is a complex job that really take practice to get even half way good at. Its not something you read a book or follow through on with simple directions. For one it takes some experiece to setup the gears properly and to read the gear wear pattern. In fact its normally a challenge to find someone in your area that knows how to work on this stuff.

The gears in the front and rear have to match.

The bottom line is that you have a large vehicle that also is a 4*4 and along with that it has a 454 engine, which basically is going to suck fuel no matter what you do. I would leave it alone or change vehicles is fuel milage is a great concern, because with that type of truck I don't see much improvemetn to be made in fuel milage.

[sub006]

I assume you have a 4-speed (overdrive) automatic trans. With 3.73 gears, you will be turning only about 2000 rpm on the highway (70 mph). This is at the bottom end of the engine's torque curve.

Going to 3.42s, your rpm drops to 1800 where the engine is much weaker, almost lugging. This cuts into the hypotherical 10% mileage improvement considerably.

Chevy engineering took all the variables of power, mileage, durability, etc. and concluded tht 3.73 was the best overall compromise. Leave it alone and save your money. Really!

[FCDGMC]

Thank you all for your responses. I have also consulted with a family member who was the head of service for a Chevrolet dealership and he concurred. He said of all the mechanics that he had working for him, only maybe two could do differential gears. I now understand that it requires careful alignment of two meshing gears (something called blueing?) That was enough.

Will focus on fixing the 02 sensor and ensuring the fuel system is at it's peak.

Thanks again.