rear diff. ratios?

[Matt400]

Im pretty sure towing in double overdrive (6th) would be very hard on the trans.

Why so? Are they making those gears less strong?

 

[wats1]

Louisana or Utah

Speed limits back east and closer to sea level

higher speed limits, elevation and weights out west

 

[wats1]

here is the difference.

Try going up a Wyoming Mountain at rated GVW in the wind.

And then,

Go up a Mountain in Lousiana at Rated GVW.

 

[Vaughn MacKenzie]

Why so? Are they making those gears less strong?

Laws of physics mostly, the higher the final ratio the more torque it takes to produce the same HP at the wheels, thus more stress on gears and lube. . . more heat and chance of breakage. Underdrive gears multiply torque, overdrive gears divide it. Double OD is . 63, that's quite a bit higher gearing than . 69 of the old auto trans and ~. 74 of manual transmissions.

 

[Matt400]

Double OD is . 63, that's quite a bit higher gearing than . 69 of the old auto trans.

True but I would think the stress load on the trans while in . 63 OD with 4. 10's would not be any more than . 69 OD with 3. 73's

Surely the steeper axle gear would make for less stress on the trans giving way for use of a taller gear.

 

[Vaughn MacKenzie]

True with the 4. 10s it should offset a lot of it Matt, and you're right, at 70mph for example the RPM is virtually identical in top gear comparing the two combos (1916 for the 4. 10s/68RFE vs 1910 RPM for the 3. 73/48RE).

But what I can see happening is guys getting the 6sp auto with 3. 73s then trying to tow in double overdrive and running at 1600 RPM & lugging the engine and stressing the trans.

 

[NIsaacs]

I know nothing about how the computerized autos work.



However, don't they downshift and eliminate lugging?



Also (for you computer experts) I have always wondered how much energy is produced and wasted by the torque converter in an auto every time you stop?



Using about a 4. 00 ratio first gear and the 4. 10 rear axle, how much torque is applied to the ground at idle and stopped?? Also, does the ECM try to keep the engine idle up and actually give it some fuel to overcome the resistance when stopped, now producing even more waste? I have no clue how to figure this out. However, it must be substantial??



If the converter generates even 2 lbs of torque at idle, that would be 32 lbs at the ground??



Nick

 

[RVTRKN]

But what I can see happening is guys getting the 6sp auto with 3. 73s then trying to tow in double overdrive and running at 1600 RPM & lugging the engine and stressing the trans.

Nothing a G56 wont cure.

 

[Matt400]

Nothing a G56 wont cure.

:-laf I will admit there has been a few times I would have liked a manual while towing but still prefer my auto for all around use. A 6sp auto with 4. 10's and selectable shift must really be nice.

 

[JHumphries]

If I had no choice but the auto, I would pick the 4. 10. That would make up for the lack of a granny gear as would the torque converter.
In my mind the best ratio for the stick is the 3. 73
If you never planned on towing anything, just moving stuff in the bed, then the 3. 42 would be great with a stick or an auto: Don't shift into or lock out the auto gears to prevent from lugging. But, when taking a cross country trip and the speed limit hits 80, let'er rip!

 

[HBarlow]

They don't, I think the focus is not wanting to run the engine at such a low rpm that efficiency falls short.

What some are overlooking is the . 625. 1 6th gear ratio, that is way tall to begin with.
Consider this chart here: Engine RPM Calculator

by scrolling down to the manual entry chart one can see that-

• 625. 1 6th gear
• 3. 42 diff ratio
• 32" tall tires

means you would have to run at 75mph just to get the engine above 1600 rpm. Throw on a little larger tire (which many do) and you would have to run at 80+ or just not use 6th at all.

By going to 4. 10's makes for much better 1st and reverse when loaded yet delivers under 1900 rpm @ 70 mph in 6th.

An excellent explanation.

My C&C with Aisin six speed automatic and 4. 10 gears runs 2100 rpm at about 75 mph. Why would I want taller differential gears? I would give up the torque multiplication provided by 4. 10 gears for heavy load starting, be forced to lock out overdrive while towing unless I wanted to tow at 70 mph, and would be lugging the engine at under 1600 rpm most of the time. I simply don't get it.

If you sit down with a truck salesman at a big truck dealer he uses the Freightliner/Navistar/whatever software to calculate gearing to cause the truck to run at the engine's most efficient/economical rpm at highway cruise speed and have adequate gearing for full loaded load starting on a grade.