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.

[C_Schomer]

I'll use an extreme example. Say the truck tops out at 100 mph and the rear is changed so it tops out at 50 mph. The truck has the same amount of gears spread out over 50 instead of 100 mph. Now let's use this percentage thing your stuck on. The example trucks top speed slowed down 50 percent so multiply that times what ever you want and the mph per gear will be 50 percent of what it was with the 100 mph gears. That's what I'm talking about being closer together. Craig

[TurnThePage]

Semantics... Change % to MPH and see what happens.

[Turtle_n_Peeps]

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

[C_Schomer]

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

[bimbert84]

rexlion wrote:

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?

The short answer is yes. It's all about final drive ratio, which includes gearing in the transmission, rear end, and torque converter. As someone else pointed out, the main difference will be in launching from a stop: since both will be in 1st gear, the rear-end ratio makes the only difference. With an 8-speed transmission, the 3.45 will have you shifting into 2nd gear halfway across the intersection. With the 3.09, it'll be 3/4 of the way across.

-- Rob

[Huntindog]

It is not all about RPM. How and where the multiplication takes place makes a difference.

If the multiplication takes place in the transmission, then everything has to work harder. The tranny itself will work harder, usually shortening it's life. The u joints, usually won't last as long as if the multiplication was in the rear end gears. Engine braking will be more effective with lower rear end gears as well which will mean that the brakes will have a easier life.

Often in other wise identical TVs, the lower geared one will have a higher tow rating because of these factors

[Highway_4x4]

My son just bought anew Ram 1500 with 3.92 gears and the hemi. Tow rating is 10K+. It is just a rocket. 70mph = just a shade under 2000rpm. My new Ram 2500 diesel is rated to tow 17,000 and with the 6 speed and 800 foot pounds of torque it will tow my 7000 pound TT easy. 3.42 gears puts 70mph at 1500 rpms.

[APT]

The 8-spd trans is really interesting. The first gear is very short for great acceleration from a start. However, I do not like turning 1500rpm at 70mph in top gear on the highway in a naturally aspirated gas engine. There isn't enough power to hold anything but flat ground. I do not know what gear choices are available with the Grand Cherokee, but I recommend driving both on the highway to compare.

[Turtle_n_Peeps]

rexlion wrote:

C Schomer wrote:
... The lower gears put the trans gears closer together and is better for creeping and reverse. Craig

I don't understand how this would happen, can someone explain further? I thought the final output ratio to the wheels would be a matter of simple multiplication between the transmission gear and the rear axle gear numbers, yielding straight line results. If changing the rear axle gear can effectively move the transmission gears' outputs closer together or further apart, that suggests a geometric progression... a parabolic curve for the results.

LOL, it doesn't that's why you don't understand it. The gears don't get closer together.

As far as your first post. You are correct. The tranny will pick the gear it needs to be in. In the years of the powerglide with 2 speeds it was important to pick a good gear for towing. Also true with the 3 speed tranny's. Less so when it got to the 4 speed and even less so when it got to the 5 speed's. With a 6 speed or above about the only thing it will effect is 1st gear starting out on a very steep hill.

All in all, with 6 speed and more gears, it doesn't really matter what rear gear you pick now days. With your jeep I would get the lowest you can get just because you might want to put big tires on it and that will throw everything out of wack.

[rexlion]

C Schomer wrote:
... The lower gears put the trans gears closer together and is better for creeping and reverse. Craig

I don't understand how this would happen, can someone explain further? I thought the final output ratio to the wheels would be a matter of simple multiplication between the transmission gear and the rear axle gear numbers, yielding straight line results. If changing the rear axle gear can effectively move the transmission gears' outputs closer together or further apart, that suggests a geometric progression... a parabolic curve for the results.

[blt2ski]

BenK wrote:
Always wondered if folks ever ride bicycles with gears and ride in hilly terrain...

Toss in a kid trailer bike (one wheel, front clamps onto your bike) or
tow a kid/cargo trailer?

Not a kid cart, but a paper cart! went slower up the hill in a lower overall gear! Lower gears only help a larger load get going faster. Top speed is dependent upon HP, Torque is what gets you going, which can be multiplied by gearing.

A rig with 400 lb ft of torque with an overal low of 24-1 with 31" tires will pull 20K lbs up a 30% grade, same motor with 16-1, will do about half that amount! This is from both experience, and cranking numbers in some formula's I have. Top speed is based on HP doing the same thing.

Axle ratio should be choosen based on tire diam, what kind of trans you have, and the rpm of the motor, in such a manner you can get the bets speed, pulling power or equal in a given gear.

Marty

[BenK]

Always wondered if folks ever ride bicycles with gears and ride in hilly terrain...

Toss in a kid trailer bike (one wheel, front clamps onto your bike) or
tow a kid/cargo trailer?

[C_Schomer]

It won't matter as long as you tow light. I'd get the lower gear in case you tow heavier some day. It won't make a hill of beans in mpg but will tow better with lower gears. The lower gears put the trans gears closer together and is better for creeping and reverse. Craig

[Ivylog]

OP because a diesel's peak torque is at a much lower rpm in my situation a 3.73 is very good but the same truck with a gas engine needs a 4.10 because of the higher rpms.
"9200lb with the 3.73, yet ~11200 with 4.10s" Other than making it a little easier to get going I would like the engineers that come up with these differences to explain it. My personal opinion is because of people that put it in D and never touch the gear shift again except to back up. Mine is a stock 02 PSD (275 hp) with 3.73 axles and I often gross over 26K in OD on flat ground. In rolling hills if I keep it above 65 I can stay in OD. On longer hills I down shift early and keep the RPMs up. I have a programmer for it but have never needed the extra HP. Even though my son has the new Ford with 400 hp/800 torque I doubt I'll pull with it as it would probably cost me a lot of money.

[bmanning]

Ivylog wrote:
When we only had a three speed tranny without OD axle ratio was very important. Unfortunately many have not figured out with just a 4 speed pulling a heavy trailer you do not need a 4.10 as keeping it out of OD and direct (the best gear to tow in) you have a 3.73. These same ones then say but off the line you need the 4.10 and with a clutch that might be true but with a torque converter... give me a break.
Not sure you need 8 as I love my PSD with a manual 6 speed. Pulling I use 5 of them unless I'm on a hill and then use the low hole to get started. Around town it becomes a 3 speed.
OP, for what you are towing I do not see any problem with a 3.09 by keeping it out of OD or on a good hill one gear lower, but you have to lift your arm and select a different gear. I would nor call that having to work.:W

Don't disagree with anything you said (and btw couldn't be more jealous of your 7.3L/handshaker 6) but it is interesting to note how much of a bump the manufacturers sometime give on tow rating due to a different axle gear.

My 2008 F250 is rated for ~9200lb with the 3.73, yet ~11200 with 4.10s. IIRC the GM twins have a similar gap, perhaps even larger.

I switched from 3.73 to 4.30 and I would do it again; it completely changed the feel and driving dynamic of my truck. It's almost as if my 5.4L whispered "thank you, this truck's heavy and I'm a little V8" LOL