Rear axle ratios and impact on towing

[rexlion]

Suppose you have a choice between two axle ratios, say they are a 3.09 and a 3.45. And let's say the vehicle has an 8 speed transmission, so there's plenty of gear flexibility. Ok, so there's roughly a 10% difference in RPM for any given gear to drive the vehicle at a certain speed. But won't the tranny simply choose a higher gear to offset the 10% difference in rear ratio, so that in the end the engine will run at pretty much the same RPM as otherwise? Or am I not understanding how it works?

Before you tell me how lousy a 3.09 is for towing, I should say that my current TT is pretty lightweight anyhow.

[transamz9]

I like my 3:42's for what I'm going to use it for. I just lock it in 5th and it runs about 1800 around 70. The short tow I did this weekend netted the same results on the lie-o-meter. When unhooked mixed driving I'm seeing 17 and hwy 20 hand calculated. I just wish they had a rear gear temp readout because the faster gear will build more heat hence the lower tow capacity.

[dodge_guy]

C Schomer wrote:
And some others aren't smart enough to get it but they show there derrieres and insult the people who do! Please disregard if you weren't one of the insulting few.

Nope, i'm with you.

[C_Schomer]

And some others aren't smart enough to get it but they show there derrieres and insult the people who do! Please disregard if you weren't one of the insulting few.

[dodge_guy]

C Schomer wrote:
BINGO! Nothing physically happens to the trans but the gears will effectively be closer together with a lower rear end cuz the final ratios all get tighter. I'm glad some people get it. Craig

Yep, you got it! That is exactly right. Some of us understand and others don't.

[C_Schomer]

BINGO! Nothing physically happens to the trans but the gears will effectively be closer together with a lower rear end cuz the final ratios all get tighter. I'm glad some people get it. Craig

[Highway_4x4]

I think it all boils down to how many feet forward the vehicle moves forward with a given number of engine revolutions.

[blt2ski]

Your in theory also has to assume that everything ELSE is equal, ie all them variables you are talking about, otherwise, it is not as close to equal to equal test as one should compare this types of things. Take a rig with say 28" tires with the 3.09, and compare it to a 3.45 with 30" tires, and you will have pretty much the same shift points at the same speeds etc. Now what will happen, is in reality, potentially the 3.09 will be a bit faster than the 3.45, as those 30" wheels and tires are bigger, heavier, take up more drivetrain HP, so will be a bit slower. Probably not a lot to worry about, but none the less, slower due to the taller diam tires.

Many ways to change out the how a given rig may do vs another one.

marty

[BenK]

Hey Turtle !!!....I've already lost all my hair so, save yours for another time.... 😉

I'll take a crack at this...

Take each component individually before adding them up....early one mentioned
if any one cycles...you can 'see' each gear that the chain is on and the other
gear that, that same chain is on...

Simple math from there dividing the gear teeth numbers...

Forget about HP and Torque for not...again why mentioned cycling...folks seem
to under stand better when they have their own skin in the game vs an inanimate
object


Gears change RPMs from one side to the other side (input to output) shafts

A 4.1 diff gear ratio will have the input pinion turn 4.1 times to *ONE* turn
of the crown gear...which is attached to the axle....which is attached to the
flange that the wheel bolts to....which will have a tire mounted on that to
connect that drive shaft turns to the pavement.

The tire is another variable, but save that for later down this thread

Hope that is clear that the drive shaft to pinion to crown gear has a ratio of
some sort that then turns the wheel

A 3.73 will have the drive shaft turn 3.73 times to *ONE* turn of the wheel

A 3.08 will have the drive shaft turn 3.08 times to *ONE* turn of the wheel

ETC



Same thing for the engine to tranny and let us us a manual instead of an automatic.
As an Automatic has a TC (torque converter that used hydraulic coupling...which
means that there is slippage in the TC and why there are now TC lockup to by
pass or remove thats slippage)

So if the the first gear is a 4.56:1, the engine will turn 4.56 times to *ONE*
turn of the tranny output shaft

And so one for the other gears in the tranny

The tranny output shaft is the drive shaft and we won't complicate this with
the mention of a transfer case and it's gears)

That drive shaft is connected to the diff input shaft, which is the pinion
from the above few paragraphs

So depending on which the gear is selected for the tranny and what the diff
ratio is...will determine the number of times the engine turns vs the wheel turns

That is the basis for all of this thread...how many times the engine turns vs
how many times the wheels turns

That is then factored by which gear ratios are selected or involved

All simple math folks



Now the variables that has folks messed up


One is gasoline vs diesel, which has different characteristics. Like that one
gallon of gasoline does NOT have as much BTU's as diesel.

Another is that one used compression ignition and the other spark. So their
componentry is different because of the stresses are different and a whole
lot more

Why, generally speaking, diesels have much more torque at lower RPMs than
gassers generally do.

Why gassers can spin to higher RPMs, generally speaking, than diesels can.



Now toss in Torque and HP to further confuse...


As for efficiency of any one gear ratio to another...it has more to
do with the pumping losses and how the gear set is designed. The
most efficient gear is a plain spur gear, but the noise from that
kind of gear would have most folks complain that something is wrong...

Everyone is used to helical gears these days, that they don't know
what a straight cut spur gear sounds like.

There is a Porsche racer who beats lots of folks and those others
have very similar 911's...he had a custom made set of straight cut
spur gears for his custom gear box. Fast, but NOISY !!!

[rexlion]

Ok, I think I get it. If I get a 3.09 instead of a 3.45, that's about a 10% higher amount of wheel turn for each rev of the engine. And if the transmission wants to shift up at a given RPM each time, in theory (all other things being held equal) my acceleration span would be 10% greater while in the given gear before the next upshift. "In theory," because the system is always sensing variables like throttle, inertial and environmental resistance, etc.

[Turtle_n_Peeps]

dodge guy wrote:

Turtle n Peeps wrote:

Each gear is more efficient.

I have no idea what is meant by this statement? :h

But the fact is, the lower the gear ratio in the rear end (higher numerically) the more horsepower that is lost to the drive train and not put to the road.

To me, that makes each gear less efficient,(talking HP) not more. 🙂

Ok, i think you are choosing to not understand how physics work!

What does that mean? :h I understand the physics of gearing just fine. I said:

But the fact is, the lower the gear ratio in the rear end (higher numerically) the more horsepower that is lost to the drive train and not put to the road.

If you think I'm incorrect with any of what I have above just say so.

If you think gearing makes more horsepower just say so.(so I can LOL):B

[dodge_guy]

Turtle n Peeps wrote:

Each gear is more efficient.

I have no idea what is meant by this statement? :h

But the fact is, the lower the gear ratio in the rear end (higher numerically) the more horsepower that is lost to the drive train and not put to the road.

To me, that makes each gear less efficient,(talking HP) not more. 🙂

Ok, i think you are choosing to not understand how physics work!

[Turtle_n_Peeps]

Each gear is more efficient.

I have no idea what is meant by this statement? :h

But the fact is, the lower the gear ratio in the rear end (higher numerically) the more horsepower that is lost to the drive train and not put to the road.

To me, that makes each gear less efficient,(talking HP) not more. 🙂

[dodge_guy]

Turtle n Peeps wrote:

C Schomer wrote:
The trans gears DO get closer together with a lower R&P! The top speed will be slower and you'll have the same number of gears so that makes the rpm change between gears closer. Mui bueno for towing! Craig

:R Oh boy!

If you have a 15% gear drop between 3rd and 4th with 2.73's you will have a 15% gear drop between 3rd and 4th with 4.56's.

The % stays the same!!!!!!!!!!!!!! Just because you have to make the gear change faster does not mean the % between gears or the ratio between the gears somehow magically changes.

This is almost as bad as people saying gears make more horsepower. :R

In the trans the spread between the gears is the same. At the wheels is where the difference is. Which is why the trans will shift faster with a lower gear ratio! Each gear is more efficient.

[Sport45]

Ma and Pa Kettle Math

🙂