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.

[45Ricochet]

Your right for the most part. These new 6-8 speed trany's make the gear ratio less of a concern than in years past. Rams Cummins with a 3.42? Fords running 3.55? and they are pulling fine. The big HP/TQ numbers also help.

[dodge_guy]

What some people fail to understand is that the torque multiplication changes with a given ratio. So with that, each gear will be that much more efficient depending on the differences between the ratios. That is why the higher the number on the ratio, the higher the towing capacity. This will always be the same even with a 20 speed transmission!

[TurnThePage]

Things have changed I know, but my particular tow vehicle has almost come to a complete stop climbing certain hills, until I regeared from 3.55 to 4.10. It was a world of difference.

[goducks10]

Biggest draw back will be acceleration thru the 1st few gears. After that the 8sp should even things out.

[MitchF150]

If one had two different rear end/tranny gear combos that would both yield the same RPM at the same highway speed, am I correct in understanding that both combos will transmit the same HP and torque, and therefore produce the same towing experience?

I forgot about the Jeep having the 8 speeds.. 🙂

As I've been schooled many times on that exact question about gear ratios, that was my thought too... Same rpms, same power output... Well, taking the 'schooling' I got, the vehicle with the lower rear gear ratio will actually have more of the engines torque being applied to the ground than the one with the higher gear ratio even running at the same rpms.

So, you could run lower rpms, but have the same or more torque going to the ground with the lower rear gear ratio and running a gear or two higher on the gear selector.

I'm sure there is a lot of math involved and I don't have a clue on that, but if you think about it when riding your old 10 speed bicycle as a kid you could change the rear wheel gears and/or the pedal gears and get a variety of different ratios that provided the 'easiest' peddling effort on your part.. I remember liking the rear wheel gear to be one of the larger dia gears and running the pedal gear on the smaller dia gear...

Anyway, there is also a difference in a diesel and a gas engine in terms of torque output at a given rpm, so that could also factor into the equation as well...

Mitch

[rexlion]

It will be a Jeep Grand Cherokee, actually. And I guess I don't truly get an axle choice per se. The diesel gets a 3.45 whereas the hemi (which I'm leaning toward) gets a 3.09. But I'm still interested in the theory behind rear ratio selection. If one had two different rear end/tranny gear combos that would both yield the same RPM at the same highway speed, am I correct in understanding that both combos will transmit the same HP and torque, and therefore produce the same towing experience?

I realize it isn't entirely that simple in prectice, because the ZF 8-speed's gear spreads are larger than 10 or 12 percent. 8th is 0.67, 7th is 0.84, 6th is 1.00, 5th is 1.29, etc. With the 3.09, what 1 gear lower will tend to do is bump the RPM up an extra 10%-20% (vs the 3.45 and the next higher gear), putting it a little higher in the power band for a given speed. Not a bad thing.

(edit): Fred, we were typing at the same time and I think you've answered my question pretty directly. Thanks. 😉

[Golden_HVAC]

Hi,

Either rear axle ratio will work fine. The truck will pick the highest gear that it can be in for a given load. So if you need 2,500 RPM to continue at a given road speed, then the truck will pick one of the 8 gear ratios and go with it.

With the 3.0:1 ratio, the truck can have a slightly lower engine speed, until it downshifts and can actually be turning faster than the 3.45 will have in the next higher gear on transmission.

So either one will make 200 HP at 3,000 RPM at say 45 MPH while going up a hill. If you need more power, either one will shift down one gear and bring up the engine RPM and increase the available HP as much as needed.

Fred.

[MitchF150]

So, you are looking at the Dodge Ram 1500 with the reference to the 8 speed trans.. 😉

For reference, I have a 13 F150 with 6 speed and 3.73 gears.

In 6th going 70 mph, it's just a tick under 2000 rpms. I do have the 20" wheels, but the overall height of the tire is only 31.9". Same as the 18" and 17" wheel options.

To compare to my my 97 F150 with the 4 speed and 3.55 gears and roughly the same dia tires, it was about the same rpms going 70 mph..

But, the 97 would have to shift down to 2nd gear going up most grades while towing a #5000 TT.

I've not towed with my 13 yet, but just driving it over the last 2 months I've owned it, I can tell you I'm gonna be glad I got the 3.73's, even towing the same #5000 TT..

I'll bet I won't be any lower than 4th gear going up those same grades... RPM wise, that will put me in the 2500-3000 range. (the 97 would be 3000-3500 rpms)

So far I'm averaging 17 mpg in my empty commuting miles (1800 miles on the clock so far).

I'd vote the the 'lowest' (higher numerically) rear gear ratio you can get and enjoy the towing experience.. 🙂

Good luck.

Mitch

[Ivylog]

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