Rear End Gear Ratio?

[Albee]

I don't think ya can, but..........Can the rear end gear ratio be changed from 3.73 to 4.10? Thought I had heard it would mess up the tranny or something. Thanks

[DmaxMaverick]

I don't think ya can, but..........Can the rear end gear ratio be changed from 3.73 to 4.10? Thought I had heard it would mess up the tranny or something. Thanks

Yes, but....
The 8.1L HD and 3500 was available with 3.73 or 4.10, so parts or a swap would be simple. You could do it, but you'd have to be creative with some custom PCM programming to correct the VSS (not critical) and ABS (very critical). Some performance tuners can adjust for the gear change, but the ABS would still be trouble. The tranny would not be effected, other than the load change.

[Albee]

OK, so i am guessing this is not anything that is fairly common to do. This is also for the 3500 DMax. So are you saying the 3500 DMax does come from the factory with an availiable 4.10 rearend? Thanks

[77TransAm]

I disagree about reprogramming the ABS. ABS sensors measure wheel speed, and in particular, difference in wheel speed. They don't care how fast the driveshafts have to spin to get the wheels rotating at those speeds.

I believe the Transmission Output Speed Signal (TOSS) is broadcast from the TCM unmolested (except for conversion from analog to digital). I would guess it is the PCM that converts this signal for the speedometer, based on the gear ratio that is programmed into that module.

It's been a few years since I've played with that stuff, though, and my memory is a bit cloudy on it.

I do know that it won't hurt the tranny a bit. Even with the higher numerical gear ratio, the input speed is still going to be limited by the engine redline, which is substantially lower than the 8.1 liter engine.

[DmaxMaverick]

All Duramax equipped 2500HD, 3500SRW and 3500DRW only have the 3.73 available. Only the 8.1L gasser has the option of 4.10 and 3.73. The 6.0L gets the 4L80 tranny and 10.5" Corporate rear diff, so not part of this equation.


The ABS WILL be an issue. The front sensors are in the hubs, and the rear sensor is in the tranny output. If you change tire size, the ABS and VSS won't see the change. Only difference will be the load change due to the tire size increase/decrease, and your actual rolling speed. The ABS, on the other hand, will see an unchanged rear speed because the gear change occurs after the VSS, but the front will see a different speed at the front because the gear change is before the sensors. If you change to 4.10, and are traveling straight forward with no traction loss, the ABS will see a slower front wheel speed at both wheels than the rear. A soon as you brake, it will *see* traction loss at both front wheels. This is why changing only tire size will not cause the ABS to fail, just changes the engagement envelope due to the actual ground speed. Changing the diff gear ratio will keep the ABS in a constant state of confusion, like the rest of us. The PCM needs to know the gear ratio, or has no way to calculate ABS function. ABS is not adaptive, and won't pick up on wheel speed changes caused by gear changes.

[77TransAm]

You're right, DMaxMaverick.

Although I did that swap several times, I was able to reprogram the TCM and ECM to do whatever I needed them to do. I shoulda stuck with what I remembered for sure: it won't hurt the tranny a bit.