3.31 gears at 40,000 lbs gross.

RoyJ wrote:

ShinerBock wrote:

RoyJ wrote:
The counterpoint being raised is given an enthusiast driver who cares about his truck (not a fleet driver), towing heavy with a 3.31 puts very little additional stress on components other than the driveshaft / u-joints.

This is where we disagree. I know that the added stress put on the driveshaft/u-joints is also placed upon the output shaft of the trans which in turn is being held to the input side of the trans via clutches. I don't see how anyone can say that there is added stress on the driveline, but not added stress on a clutch and output shaft that is connected to that same driveline.

I'll start by saying we're talking RV use, NOT commercial under-speed cases. In the latter case you would have higher average torque in the transmission input side, because you're purposely lowering the cruising speed. So with a fix cruising hp requirement: we drop the rpm by 25%, up goes engine torque by 25%.

In the RV case, we're locking out 9th / 10th. So if cruising rpm is identical, engine output HAS to be identical. Engine does not know what rear end there is, only the total gear reduction ratio.

If you look at my chart earlier, both 3.31 and 4.30 truck engines are putting out ~420 hp and ~800 lb-ft of torque. The only difference is the 3.31 truck in 4th, and 4.30 truck in 6th. The flywheel / TC clutch would not know the difference.

Again, I am not talking about those who manually lock out gears in order to reduce the added stress placed upon the driveline and overdrive clutches(that are directly attached to the said driveline). I am talking about those who let the computer do all the work and the computer will keep the transmission in these higher gears (as seen in the video) even when towing more weight than its rating.

Hence the reason why the fact still stands that taller rear gears put more stress on the trans and driveline gear for gear than a shorter rear gear. Gear for gear, not one 8th and the other in 10th. Gear for gear.

There is a reason that many trans manufacturers and off-road enthusiasts say you should regear your vehicles when you put larger tires that alter your final drive ratio. It puts more stress on your drivetrain(including trans) and causes premature trans failure due to the taller gearing pulling heavier weight so you need to regear to reduce this stress.

"Increasing Tire or Rim Size Can Cause Premature Wear
You Don’t Want To Reduce Your Vehicle’s Effective Gear Ratio
The increased size causes the tires to rotate slower to cover the same amount of ground. It will take more power to accelerate because your drivetrain’s mechanical advantage has been decreased. If you are running an automotive transmission this can cause some serious strain depending on the current gear ratio if you choose not to re-gear your system."

INCREASING TIRE SIZE CAN WEAR OUT YOUR DRIVETRAIN & TRANSMISSION

"This theory works against you, however, when you increase the radius of a tire, because it now requires more torque from your driveline to turn that larger tire. This generally does not have much of a negative effect on the engine components themselves, although it will now require the engine to work harder to maintain the same speed as before. The transmission components, on the other hand, especially the clutches and bands, take the brunt of the abuse. Clutches and bands will begin to create excessive heat as they struggle to operate under higher constant stress. Excessive heat cycles will cause the friction surfaces to start slipping, and at that point the transmission will need to be overhauled. If the core problem of the failure is not addressed — increased tire size — problems will occur again and again."

Transmissions & Oversized Tires: How Changing Gear Ratios Helps Prevent Drivetrain Damage

"Q: Are there other issues beyond part fatigue to consider when you upgrade to bigger tires?

A: Absolutely. A gear ratio change is essential to achieving a good balance between the torque you want, and the efficiency of your engine and transmission. The larger tire size changes the optimization of the transmission-to-engine operation. You need to change the axle’s gear ratio, or you’ll get terrible gas mileage and wear out the transmission quicker."

Spicer-Bigger Tires Demand Related Modifications