Changing from 3.73 to 4.10 gears

4x4ord wrote:

Turtle n Peeps wrote:

noteven wrote:
Anyone know why pickup OEM’s give higher trailer weight ratings with a spec change to lower rear axle ratio only?

It's about duty cycle. The more gear a truck, tractor or motorcycle has the more duty cycle it has.

My tractor has a gear into a gear into a gear. Wide open in low it will go an amazing 1/2 MPH but it puts down tens of thousands of ft/lbs to the ground.

The formula is this for 1 horse power: Lifting 33,000 lbs one foot in one minute. Now a horse can't lift 33,000 lbs without "help." So if the help comes in the form of gears or block and tackle or anything that will "help" the horse lift lets say 500 lbs 66 feet in one minute. (I think I did the math right on that?) :R

It doesn't matter if that horse can lift 33,000 lbs one foot in a minute or if that same horse lifts 500 lbs in 66 feet in a minute or any other combination..............that horse is still putting out 1 horse power.

That is all a gear is doing with a tow rig or my race car or whatever...........it's still putting out the exact same HP to the ground........same as the horse example above.

So part of the reason OEMs give higher trailer ratings to truck's with lower speed gear ratios is, as T and P points out, the torque multiplication to get the load moving but I believe an even more important aspect of specing the right gear ratio is to start with an assumption of how the truck will be used including the weight of the load, the grades to be climbed and the power available. Say a 25,000 lb diesel truck/trailer with a 440 HP engine is being speced to pull an RV down highways with 6% grades and 70 mph speed limits. It can be determined that 440 is capable of moving a 25000 lb load along at 65 mph going up a 6% grade. The ideal gear ratio for this application would cause the transmission to shift into direct or closest gear available to direct (4th) and allow the engine to run close to the rpm where maximum power is available (2800 rpm). The 3.55 ratio in my truck puts me at 69 mph at 2800 rpm.

If the same truck is expected to pull 35000 lbs down that same highway it can be determined that 440 HP can only pull the hill at about 52 mph. It would be desirable for the transmission to again be in 4th gear while the engine is running near 2800 rpm.

This truck will have a little smaller diameter tires and should have a 4.30 rear gear ratio.

^^^^ Yep! Just like my horse example above, big trucks get more gear because they will haul very heavy loads and need the gear to "lift the load" just like the horse. That horse is still just "one horse power" but the gears or block and tackle give that 1 horse power the ability to lift 33,000 lbs at the expense of time OR distance.

When it's at the expense of time OR distance power will remain the same.