Great article on gear ratios

[i82much]

Some very insightful comments from a GM engineer on gearing, particularly the L96/6l90 combo:

 

http://www.forconstructionpros.com/article/10288893/how-to-pick-pickup-truck-axle-gear-ratios-to-improve-performance

 

Interesting that the GM engineer seems to emphasize thermal considerations more than actual pulling power, particularly the bit about the 4.10 axle being able to shift into 3rd gear sooner and allow for lower under hood temperatures.

 

I am really, really glad GM made the decision to have 4.10 gears standard in these trucks. I was only going to buy a relatively inexpensive gasser leftover truck, and before 4.10's were standard the chances were pretty good the leftovers were going to be 3.73 trucks. It was a lot easier to find what I wanted now that the locker and 4.10 come standard.

[Cowpie]

It is realistic within the extremely limited options available in the pickups offered by the OEM's. A taller diff ratio would work just as well if they had more options on transmission gear ratios. Having two overdrive gears in a 6 speed is pretty bad. It works in this setup, but is pretty lame in concept. I have 2 overdive gears in the 18 speed on my commercial truck, and hardly ever use either one, even running 70 mph down the road, so not sure why having a deeper reduction bottom and 1 overdrive gear wouldn't have been better for GM to use. Overdrive gears are inherently less efficient than direct or lower. A large majority of heavy commercial truck OEM's are dumping overdrive transmissions right and left as their base transmission offering and going with taller diff ratios and direct drive top gear transmissions. But they also allow the buyer to spec alternative transmissions and diff ratios.

 

When one is stuck with only 1 transmission with each engine, with no option for a different transmission, that leaves little wiggle room for diff ratios to get maximum efficiency and performance to meet the capability of the pickup that it is marketed for.

[i82much]

i think the issue is the physical size of the gears. steeper gears are either weaker or bigger, so i suspect fitment is the issue.

[Cowpie]

Taller gears are not necessarily weaker than higher ratio gears. It has more to do with torque stresses on the drivetrain and having the right transmission gear ratios to go along with it. There are heavy commercial trucks that use gear ratios clear down to 2.41 and direct drive transmissions that move 80,000 lb of truck and cargo nationwide. But the drivelines are spec'd to deal with some very severe stresses going on. It is more efficient to run in direct than overdrives, and fuel economy issues are why they do that. But they have a wide range of transmissions, drivelines, axles and such to fit a perfect setup together. Done improperly, and torsional vibrations and torque stress can really take components apart. It is unrealistic to have that wide variance of options for a OEM making pickups. We only get one transmission, and the diff ratio choices are limited but have to cover a wide range of uses. Mass production on that scale saves money to the consumer, but also severely limits choices.

 

Either way. I have found over many years that the 4.10 is just about the best ratio for a pickup that has to be taken to it's physical limits. Not the most ideal for a grocery getting, soccer mom kind of pickup, as it is not going to be the most fuel efficient ratio. But for the class 2/3 pickups of this forum thread, it is a darn good overall ratio.

[i82much]

The 4.10's only really hurt you unloaded at freeway speeds. Any other time, the performance is better with the 4.10's and the mileage is just as good.

 

Steeper gears are weaker, not taller gears.

[Cowpie]

Not convinced that either gears are weaker. Components are made from the same material on the same production line using the same grinding processes.

[XSKIER]

Given a fixed axle housing size, not comparing semis to haf tons, the 4.56s will be weaker than the 3.08s. In order to fit in the axle housing, the pinion of the 4.56 has to be way smaller in diameter and gear tooth size. You can't make a 8.6 inch ring gear any larger diameter, so the pinion has to get smaller.

[i82much]

Not convinced that either gears are weaker. Components are made from the same material on the same production line using the same grinding processes.

 

yes but think about how the size relationship of the gears has to change with the ratio.

[sdeeter19555]

Given a fixed axle housing size, not comparing semis to haf tons, the 4.56s will be weaker than the 3.08s. In order to fit in the axle housing, the pinion of the 4.56 has to be way smaller in diameter and gear tooth size. You can't make a 8.6 inch ring gear any larger diameter, so the pinion has to get smaller.

This is why only certain ratios are offered in certain axles...can only get so small of a pinion...

 

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[i82much]

i think we are talking about transmission ratios but principle is the same.

[4x4Pete]

The overdrive helps....without it a 4.10 would be crazy at highway speeds. A overdrive of.7 or so makes a 4.10 similar to 3.42 with a non overdrive transmission. It has to help with mileage it just becomes a question of how much torque do you have? My old Pontiac would have no problem with an overdrive trans, it feels like it runs out of gear not power because Pontiacs have gobs of torque. The newer truck motors have a nice flat torque curve and manage the overdrive and 4.10 gears nicely.

[Cowpie]

This is why only certain ratios are offered in certain axles...can only get so small of a pinion...

 

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Probably true, but that dimensional size does not necessary equate to weaker. Metallurgy determines that. There is no evidence that larger ratio gearing is weaker than it's lower ratio cousins. The torsional stresses are also significantly different. There is far less torsional stress on a higher ratio gear set than a smaller ratio. As the ratio of driveshaft rotation compared to drive axle rotation decreases, torsional stresses increase. So a smaller pinion gear on a higher ratio differential does not necessarily equate to weaker. And the same principle applies to pickups, cars, or even heavy commercial trucks. The same size differential housing on the typical heavy truck is used from ratios of 2.28 all the way to 7.17. So the assertion that pickups are soooo much different because they have one size differential housing is moot. It is just trying to deflect the discussion and exposes a lack of understanding.

 

This is why, typically, a pickup with a higher ratio rear will have a higher towing capacity... the torsional stresses are significantly less and the components can hold up better under heavier applications.

[sdeeter19555]

The diameter of the pinion becomes so small that causes it to be weaker than its lower numerical cousin. Not to mention the ring gear becomes super thin on excessively numerically low gearsets.

 

And heavy truck doesn't not equal pickup...differential size is everything. The smaller ring gears have limitations...you can only adjust the carrier side to side so much...

 

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[i82much]

 

 

Probably true, but that dimensional size does not necessary equate to weaker. Metallurgy determines that. There is no evidence that larger ratio gearing is weaker than it's lower ratio cousins. The torsional stresses are also significantly different. There is far less torsional stress on a higher ratio gear set than a smaller ratio. As the ratio of driveshaft rotation compared to drive axle rotation decreases, torsional stresses increase. So a smaller pinion gear on a higher ratio differential does not necessarily equate to weaker. And the same principle applies to pickups, cars, or even heavy commercial trucks. The same size differential housing on the typical heavy truck is used from ratios of 2.28 all the way to 7.17. So the assertion that pickups are soooo much different because they have one size differential housing is moot. It is just trying to deflect the discussion and exposes a lack of understanding.

 

This is why, typically, a pickup with a higher ratio rear will have a higher towing capacity... the torsional stresses are significantly less and the components can hold up better under heavier applications.

Sorry, I've enjoyed many of your posts and I hate to say this, but I am quite certain that this post is a work of fiction.

 

It doesn't matter what type of mechanical gear you have, a ring & pinion, chain & sprocket, a CVT, whatever. You fix the size of one gear, and the other gear has to change size in order to change the ratio. There is only so much you can do within the constraints of a given axle housing, transmission. Same is true for timing chains, engine pulleys, motorcycle drive chains, lawnmower belts, etc.

 

Archimedes didn't say he could move the world with a lever that would fit into a transmission case.

[i82much]

I would say, however, that I do not know whether gear sizing constraints prompted GM to use a transmission that does not have a direct drive. That was a mere supposition on my part, just to clarify.