Regear or buy another truck

Homewrecker007 · February 22, 2023

Here's the speed and rpm changes with the gear changes with the 6l80E transmission

16LT4 · February 23, 2023

On 2/22/2023 at 4:30 AM, Homewrecker007 said:

Best Decision I ever made. There is a slight mpg drop but not bad. Before the gear change, trucks were struggling to keep up with traffic pulling a trailer and constantly running at 5000 rpm. Swapped in the 3.73's now it will do 70-75 no problem

I'm interested in the slight mpg drop: do you notice this across a tank (on average), around town, on the highway, towing, etc? All else being equal, I'd expect to see a possible drop on the highway, but with a slight gain around town; the affect on the average being of course based on which driving environment the truck sees more. Can you quantify what "slight" is?

As much as I would like 3.73s, I know I (most likely) won't be doing it due to my time (too many other projects) and that my truck is what we use predominately to run NY-OBX a few times a year, so I want to eek out whatever mileage I can. In a few years, we'll be looking at today through rose-colored glasses, remembering the "days of cheap fuel, when we didn't really care about mileage." I just like to see the numbers and data, especially on the gear ratio experiment. My '04 FX4 had 3.73s, but also tires that were upsized one sidewall size, and I could feel the difference accelerating from a stop. With only a 4 speed, the upsize was done not only for looks, but also to drop engine RPM on the highway a little, but I could feel it with a trailer.

Homewrecker007 · February 24, 2023

18 hours ago, 16LT4 said:

I'm interested in the slight mpg drop: do you notice this across a tank (on average), around town, on the highway, towing, etc? All else being equal, I'd expect to see a possible drop on the highway, but with a slight gain around town; the affect on the average being of course based on which driving environment the truck sees more. Can you quantify what "slight" is?

As much as I would like 3.73s, I know I (most likely) won't be doing it due to my time (too many other projects) and that my truck is what we use predominately to run NY-OBX a few times a year, so I want to eek out whatever mileage I can. In a few years, we'll be looking at today through rose-colored glasses, remembering the "days of cheap fuel, when we didn't really care about mileage." I just like to see the numbers and data, especially on the gear ratio experiment. My '04 FX4 had 3.73s, but also tires that were upsized one sidewall size, and I could feel the difference accelerating from a stop. With only a 4 speed, the upsize was done not only for looks, but also to drop engine RPM on the highway a little, but I could feel it with a trailer.

With a trailer on I get garbage mileage around 5-7mpg slower driving 55-60mph and pulling my 1929 model A I can get 10mpg. I also turned off the DOD and VVT that alone killed the mpgs, but I haven't had a single issue with engine or transmission in over 220,000 miles. I have the 5.3l and 6l80E with stock size tires also runninga pedal commander, it doesn't add power but it does make takeoffs and passing more aggressive which costs some fuel. I'm use to it so i easy in to the throttle, but when the wife drives the first few takeoffs she tries to launch us to the moon. Running empty driving sensible on mostly interstate I get 15 to 18 mpg back roads and hills, city or just plain foot in it all the time 12.5 to 14mpg

XMR · March 5, 2023

Found some pull out 3.73 front diff and rear end for a reasonable price, any part numbers I should be looking for to confirm they’re 3.73s. Also found some 35x11.5r20s that are like 17 lbs lighter per tire than my 35x12.5r20 toyo m/t’s that should help.

Homewrecker007 · March 6, 2023

The opt code is GT4 if they are still with the vehicle.

Blk2015gmc · March 10, 2023

Just for reference. My 2015 5.3 with a 6 speed is lifted on 37's and I went 4.56 gears front and back. Its perfect. I'm also tuned for premium with headers and full exhaust. I'm pretty certain if I had 35's I'd do 4.10s. My buddy who lowered his Silverado 5.3 and went with 22 inch rims and 32s or 33s did 4.10s and he loved it. 4.10s will have a bit more snap out of the hole but will rev a bit higher at highway speeds than 3.73s it all depends what you want. I can drive no problem at 70-80 mph. Gas mileage is a different story at those speeds though lol as my trucks a brick on the highway.

Spurshot · March 10, 2023

On 2/12/2023 at 7:14 PM, XMR said:

18 Sierra sle with 5.3 think it’s 3.08 gears, 6” lift 35x12.5r20. First time buying a Gm truck and I’m starting to regret moving from ford. My 18 f150 with a 5.0 10speed and 35s had no issue towing or driving. This truck has issues doing even 70-75. I just deal with that but towing my canam defender is seriously depressing. Can barely do 65, 66-75 and it’s running 3k rpm and will not upshift what’s so ever, for an around 4500 load with the trailer that’s kind of ridiculous. I would like to be doing 75-80, but usually we run 2 sxs’s per truck which I won’t even attempt right now. I’ve had the truck since November, can’t go smaller on the tires. I actually use the truck off-road going to parks and getting around my buddies property where we keep the bikes. Should I even bother with spending the money on a regear or should I just bite the bullet and buy another truck? Other than the lack of power the truck seems to not mind the load. But I’m not trying to be limited to like 55 mph with two sxs’s on the trailer. Did I buy the wrong truck? Btw trucks in Texas, my local highway is 75 mph. I’ve thought about just buying and old diesel as a tow pig but I have enough stuff to work on, don’t feel like maintaining another vehicle. I would like to keep the truck, it’s nice to have a lighter 1/2 ton in the group.

I have a 2014 Sierra 1500 crew 4x4 12.50x33" tires with 4.10 gears 5.3L and a 6 speed automatic. I've flat towed my 4500lb Jeep from LA to Idaho at speeds up to 75mph for many hours at a time. Not sure why your's seems handicapped. You might have the engine checked. But the gearing could be it. The 35s and lift also detract from the highway performance. The 3.08 gears are pretty far off the mark for 35s and a lift, let alone towing.

Edited March 10, 2023 by Spurshot

tannerga · March 15, 2023

4.10s is probably the answer with 35s as these guys are saying. Did 4.56s and it's a little extra compromised on the highway.

MikeBMW · March 16, 2023

I have the GU6 - REAR AXLE 3.42 RATIO and tuned the speedometer for the 35s.
It runs as well as when I had the stock tires.
No issues on the highways and, shh, I've had it over a hundred a time or two.

Edited March 16, 2023 by MikeBMW

Homewrecker007 · March 16, 2023

1 hour ago, MikeBMW said:

I have the GU6 - REAR AXLE 3.42 RATIO and tuned the speedometer for the 35s.
It runs as well as when I had the stock tires.
No issues on the highways and, shh, I've had it over a hundred a time or two.

Have you had any drive shaft issues or did you swap it out already?

jonathanspw · March 16, 2023

I have 3.08s right now and I have 4.10s on the way. I'll let you know how it goes.

MikeBMW · March 16, 2023

14 hours ago, Homewrecker007 said:

Have you had any drive shaft issues or did you swap it out already?

Stock drive shaft - no issues whatsoever.

M1ck3y · March 31, 2023

On 3/16/2023 at 12:50 AM, Homewrecker007 said:

Have you had any drive shaft issues or did you swap it out already?

Justin with Blackbear performance told me the stock driveshaft is good to 130ish.

I should say that 130ish is for the 8spds with 3.23 gearing.

Edited March 31, 2023 by M1ck3y

AndrewD · April 16

Honestly I'm baffled that gm would put out a "Z71" with 3.08 gears. Mine is a 2015 Z71 w/the G80 rear end. It came w/3.42's and I run 35'12.5 18's on a 3.5" lift and haven't had any issues. I don't tow a lot but when I do it never has an issue.