Question about gearing

[HaulinBass02]

So we finally took a long trip with our new rig, which performed better than expected. Coming from a diesel tow rig to a gas tow rig was a bit of a culture shock; the high revving 6.0L does the job but it is a little unnerving sometimes! I don't have a choice unless I could afford a Duraburb conversion (which at this time I can't)
I do know that I could benefit from better gearing to help the Suburban out.

And before anyone suggests it, NO, going down to factory sized tires is not an option. We do too much trail riding which I need the bigger tires (33" for better clearance.

My question is this, is there a mechanical difference i.e. more or less work/HP used, if the engine is having to turn at a given RPM for a certain speed in one gear vs another?

Example: 2700RPM in 5th gear (0.85:1 OD) vs turning the same RPM in 4th gear (1.15:1) at the same wheel speed (65mph). I'm not sure how to phrase this or explain it, but I'm wondering if the engine is having to work as hard if it is in a higher transmission gear vs a lower gear?

I found that the Suburban will cruise at about 65 mph at about ~2000 RPM fairly well, but I also know that the power curve for the 6.0L climbs throughout the RPM range. It make more HP at higher RPMs. So I know that it will cruise better with less downshifting at higher RPM. I'm just trying to find that optimal combo.

I'm thinking that if I can get the cruise RPM to about ~2300 in 5th gear at 65 mph would be great. That puts the 4th downshift to ~3150 or so and it climbs well like that! 4.10 seems to be what'll put me there. That would even allow me to tow some in 6th gear too at about 1800-1900 RPM.

Help in answering my initial question will give me some insight to select the right rear end gear for my combo.

I'm basically debating on whether or not going from 3.73 to a 4.10 is good or if 4.56 is THAT much better. Especially given that we do drive the Suburban on road trips without the trailer, I'm considering the 4.10s as a good intermediate.

[APT]

65mph is around 2200rpm in 5th gear, and 3000rpm in 4th gear. I use 5th for flat highway and let the speed drop up small hills and overpasses. When I lose 10mph, I drop a gear and hold speed until flat or downhill allows gain back to my target speed. If it is a hilly area or stronger headwinds, I just limit top gear to 4th and cruise at 3000rpm. Wife and kids don't care. It isn't very loud at 3000rpm and part throttle. It rarely downshifts. The engine will do that for 300k miles.

[blt2ski]

BEST power per say for your motor is where the torque and hp curves cross in Bens Diagrams. ABout 2500-2800 as noted. "IF" this were me, I would shoot for this rpm in 5th, and then in the 3000-4000 rpm range in 4th direct.

As far as which gearing is better......neither, or as it is sometimes called, "IT DEPENDS" My navistar is a direct drive trans, I have to use direct as some use OD. I also need a much lower first gear to get me going vs if using an OD trans. One time I compared a 3.42 geared rig with a 4sp direct drive manual, and a 4.10 geared 5 sp manual. Generally speaking, both had the same overall low, same overall high ratio rpm at 60. As in old days, "gear it fast, run it slow" ie a 3.08 gear ratio in direct, or "gear it slow, run it fast" ie a 4.10 gear rig with a .75 OD is doing the same rpm at 60, 65 etc.

Chose poison. At the end of the day, the best ratio is based on tire diam, speeds you drive, and rpm that is best for YOUR engine at those speeds. This is what the gear ratio should be for, not a bogus "engine/gear ratio" tow rating.

Marty

[time2roll]

You will not lose any economy when towing. More likely to gain efficiency as your engine is more efficient when running at the proper RPM to make the power needed. Currently at 2000 rpm you are in effect lugging the motor as it is very low on the power curves (see BenK above).

Low RPM efficiency is for low profile econobox cars not pulling RVs down the road.

[HaulinBass02]

time2roll wrote:
If you are already at 3.73 I would skip 4.10 and go 4.56 or 4.88

This calculator can adjust all ratio, tires, transmission, even transfer case to find where you are and where you will end up:
http://4lo.com/calc/gearratio.php

I would aim more toward cruising at 2500-2700 and climbing at 3300-3600 rpm.

In looking at the power curves I think the sweet spot at 2500 is where I need to be, I'm just hesitant at the loss of mileage in cruising at that high of an RPM..... I have no way of knowing if the decreased workload on the engine would actually help economy or not. I can only hope that it would.
I guess as long as I'm not getting any worse than 7.5mpg I'm doing good right?

[HaulinBass02]

For some reason I can't quote you Kjun, but in response to your earlier post, I understand what you are getting at but isn't this actually backwards? An OD ratio is supposed to be more efficient from decreased power? An underdriven gear (1.1:1 or higher) should increase wheel power while an overdriven gear (.99:1 or lower) should reduce power to the wheels?

[Kjun]

try this for tire size calculations TireSize.com

[HaulinBass02]

BenK wrote:

HaulinBass02 wrote:

I found that the Suburban will cruise at about 65 mph at about ~2000 RPM fairly well, but I also know that the power curve for the 6.0L climbs throughout the RPM range. It make more HP at higher RPMs. So I know that it will cruise better with less downshifting at higher RPM. I'm just trying to find that optimal combo.

I'm thinking that if I can get the cruise RPM to about ~2300 in 5th gear at 65 mph would be great. That puts the 4th downshift to ~3150 or so and it climbs well like that! 4.10 seems to be what'll put me there. That would even allow me to tow some in 6th gear too at about 1800-1900 RPM.

That is what the "Tow/Haul" function is for and assume your Sub has it, right?

Tow/Haul will change the computer look up tables for shifting. It is
married with the torque management system

Not sure I'm understanding the answer. The T/H changes the torque converter lockup (keeps it locked more and quicker) and does change the shift tables from non T/H. It still shifts when it needs to and doesn't hold a gear any longer under load than if I didn't have it in T/H. What I was getting at was if I put the cruise RPM higher in the band, preferably around 2500 RPM (which seems to be the sweet spot) then it should have best combo of TQ and HP which "should" help it from stop down shifting to hold speed.

Help in answering my initial question will give me some insight to select the right rear end gear for my combo.

I'm basically debating on whether or not going from 3.73 to a 4.10 is good or if 4.56 is THAT much better. Especially given that we do drive the Suburban on road trips without the trailer, I'm considering the 4.10s as a good intermediate.

Look at my sig...ordered 4.1's for my Sub. Yours has the double OD,
so better situation than mine...

Does your sub have the 4L80E? I've been running and comparing numbers from several different truck/trans/gear combos and it seems that for a part-time tow vehicle going back to the 4.10s are what I need to be looking at.

[Kjun]

I think I follow your question. Most power curves are a 1 to 1 ratio. When you start applying power curves to gear ratios anything under 1 to 1 increases the rear wheel power and any thing over 1 to 1 decreases power. Its going to take a lot more power in overdrive. A 20% overdrive will reduce the power curve HP and torque at the same engine RPM.

[time2roll]

If you are already at 3.73 I would skip 4.10 and go 4.56 or 4.88

This calculator can adjust all ratio, tires, transmission, even transfer case to find where you are and where you will end up:
http://4lo.com/calc/gearratio.php

I would aim more toward cruising at 2500-2700 and climbing at 3300-3600 rpm.

[Old-Biscuit]

3:73s would have been fine if it wasn't for the tall tires

4:10s with the 33" tires will get you back to better use of power

[BenK]

Here are some torque/HP curves for GM's 6.0L 2010 and 2012 for their
express Savana vans...SUV's should have a bit more but close for discussion
purposes (IMHO)


GM6.0L.2010




This is for the Sierra Pickup and should be very close to the SUV version

HaulinBass02 wrote:
So we finally took a long trip with our new rig, which performed better than expected. Coming from a diesel tow rig to a gas tow rig was a bit of a culture shock; the high revving 6.0L does the job but it is a little unnerving sometimes! I don't have a choice unless I could afford a Duraburb conversion (which at this time I can't)
I do know that I could benefit from better gearing to help the Suburban out.

And before anyone suggests it, NO, going down to factory sized tires is not an option. We do too much trail riding which I need the bigger tires (33" for better clearance.

Normal OEM tire dia used to be 30" and with the larger dia wheels, some
are 31" to 32"...or more

With the 30" OEM dia, going to a 33" will have a diff gear ratio reduction
of about 10% less torque multiplication

Reference the above curves and see how that 10% factors the torque
multiplication to the drive axle...then factor the tire dia difference
Better to find the tire rev's per mile, as that is a more accurate
reference spec

My question is this, is there a mechanical difference i.e. more or less work/HP used, if the engine is having to turn at a given RPM for a certain speed in one gear vs another?

Example: 2700RPM in 5th gear (0.85:1 OD) vs turning the same RPM in 4th gear (1.15:1) at the same wheel speed (65mph). I'm not sure how to phrase this or explain it, but I'm wondering if the engine is having to work as hard if it is in a higher transmission gear vs a lower gear?

Yes, the engine is working harder, as the torque multiplication is less in OD. By the difference in gear ratio

I found that the Suburban will cruise at about 65 mph at about ~2000 RPM fairly well, but I also know that the power curve for the 6.0L climbs throughout the RPM range. It make more HP at higher RPMs. So I know that it will cruise better with less downshifting at higher RPM. I'm just trying to find that optimal combo.


I'm thinking that if I can get the cruise RPM to about ~2300 in 5th gear at 65 mph would be great. That puts the 4th downshift to ~3150 or so and it climbs well like that! 4.10 seems to be what'll put me there. That would even allow me to tow some in 6th gear too at about 1800-1900 RPM.

That is what the "Tow/Haul" function is for and assume your Sub has it, right?

Tow/Haul will change the computer look up tables for shifting. It is
married with the torque management system

Help in answering my initial question will give me some insight to select the right rear end gear for my combo.

I'm basically debating on whether or not going from 3.73 to a 4.10 is good or if 4.56 is THAT much better. Especially given that we do drive the Suburban on road trips without the trailer, I'm considering the 4.10s as a good intermediate.

Look at my sig...ordered 4.1's for my Sub. Yours has the double OD,
so better situation than mine...