Changing Diff gears

marpel · June 19, 2014

Vehicle is 2012 Sierra, 6.2L, 3:73, heavy duty towing package. Rec. trailer is about 4500 lbs loaded.

The vehicle tows just fine up to about 55-60 mph and I can tow in OD (17-1800 RPMs), without T/H up to that speed. However, I find when I start to approach 65, if I leave it in OD, it will drop down into 5th at the slightest incline or head wind and the RPMs in 5th will be 2250-2300. Although I don't necessarily mind 5th and those RPMs, the majority of times we travel, we are driving from B.C. to Denver, Co to see the family, so would like to mitigate the 5th/2300 RPMs a bit. My pseudo mechanic brother in law suggests I would be able to somewhat solve this issue by going to 3:55s (I figured I should go the other way to 4:10s but I know very little about these things).

So, a couple questions - Do you agree with B-I-L? If so, presumably the overall tow capability would drop somewhat (currently rated for 10,400 lbs.)? And, is there a formula for determining how the tow rating would change, based on changing the Diff gears? It seems there is a bit of a gap between 1800 and 2300 RPMs that is not being utilized to tow my particular load.

Any other comments/suggestions would be appreciated.

Thanks

sdeeter19555 · June 20, 2014

Couple things...

First, you are correct. The next logical step in gearing for towing would be 4.10s. However...

By changing the differential gearing, its essentially the same as you manually shifting into 5th (not calculating RPMs, just a generalization). In other words 3.73s & 5th would be similar to 4.10s & 6th. If you swap differential gearing, that's more/less permanent; so imagine driving around everywhere (empty and loaded, towing and not towing) in 5th gear...would you want that? You would be further ahead (in my opinion) to simply pull the trans into 5th, to minimize shifting, when you are towing; and enjoy the mileage and driveability of the 3.73s when unloaded.

Again, that's just my opinion...

00Silverado4x4 · June 20, 2014

Couple things...

First, you are correct. The next logical step in gearing for towing would be 4.10s. However...

By changing the differential gearing, its essentially the same as you manually shifting into 5th (not calculating RPMs, just a generalization). In other words 3.73s & 5th would be similar to 4.10s & 6th. If you swap differential gearing, that's more/less permanent; so imagine driving around everywhere (empty and loaded, towing and not towing) in 5th gear...would you want that? You would be further ahead (in my opinion) to simply pull the trans into 5th, to minimize shifting, when you are towing; and enjoy the mileage and driveability of the 3.73s when unloaded.

Again, that's just my opinion...

Agreed, 3.55 gears would only make the engine down shift more often and it will put less torque to the wheels. Also 2300rpm is actually really good as far as towing and MPGs. When I tow I put my truck (4spd) down in 3rd and tow/haul activated. I can still get 11-14 mpg as long I'm not trying to maintain 70+ mph.

marpel · June 21, 2014

Steve and Mark,

Thanks for your replies. Not that I would actually change out the gearing, but thought I would check to see if the B-I-L was on the correct track.

Still, it does seem a bit of a shame that the truck is just a smidge too weak (at highway speeds - on non-highway roads it can tow a house with no issues) in 6th at 17/1800 rpms and a bit overkill in 5th at 2300. If I could find a compromise gearing that would allow 20/2100 rpms, that would be optimal in my opinion.

Thanks again,

Marv

sdeeter19555 · June 21, 2014

You need to ask yourself just how much you actually tow...is it worth the expense?

Sent from my SAMSUNG-SM-G730A using Tapatalk

kstruckcountry · June 21, 2014

Towing in 5th at 2300 RPM's is what your truck is designed to do... That is not an extreme engine speed by any means. I always bump down a gear towing and in the mountains, and I still get good mileage. Even with lower gears your truck will still want to rev. Lock it in 5th and go. If you want to tow at really low rpm, you need a diesel.

Danthaman69 · June 22, 2014

Towing in 5th at 2300 RPM's is what your truck is designed to do... That is not an extreme engine speed by any means. I always bump down a gear towing and in the mountains, and I still get good mileage. Even with lower gears your truck will still want to rev. Lock it in 5th and go. If you want to tow at really low rpm, you need a diesel.

What He Said! Diesel And Be Done With it.