Question about gearing

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...