What did gm use as a factory gear, i hear people say both ratio's but i didn't think they would use two so close together...

Vehicle in question is a 06 c1500 w/4.8 does 78MPH at 2000 RPM

none of the codes in the glove box tell the gears...

 

One of codes should tell you. They had 3.23 3.42 3.73 and 4.10 i believe.

 

my 03 5.3 auto with the 255/70 16 stock tires came with 3.42's i believe this is the most common gear, and the NNBS trucks started showing up with 3.23 to increase MPG, but im not exactly sure. there is an RPO code in the glovebox that will tell you what ratio you have. It will start with a G.

 

my 05 has 3.23s

 

gt4 = 3.73

 

Gq1 -- axle rear, std ratio
gt4 -- axle rear, 3.73 ratio
gt5 -- axle rear, 4.10 ratio
gu4 -- axle rear, 3.08 ratio
gu5 -- axle rear, 3.23 ratio
gu6 -- axle rear, 3.42 ratio
g80 -- axle rear, limited slip (positraction

 

found it! GU5 - 3.23 then, someone here had said a HC8 for 3.23, no wonder i didn't see it before...

 

If you find codes in a glovebox; axle code will be revealed.

As far as ratios so close together. 3.42s are fairly common in general. It was std. gear used by GM til '04 IIRC; 3.23s then became std. gear in general. Yes, it was done by manf. for mileage.
3.73 was sometimes spec'd by individual dealer. I've seen some dealers & almost every truck had a 3.73. For $50 upcharge, not bad from a performance standpoint.
The 3.08 is a highway/"mileage" gear & if I originally had that, 3.42/or 3.73 would already be in there.
Same for 3.23 HWY.
3.42 is what I have & to me, it's the pefect gear. Best of both MPG & performance. I can't see myself going to anything higher than a 3.73.

3.73/ 4.10 performance/ towing.

I was getting ready to post earlier, but somebody just had to bother me to use computer when I was about to reply. I see he already told ya.

 

thank, ya ona different post, someone had different codes for the gears and i couldn't find any on my truck that matched but then i found the gu5, this is a better list..

ya, i think the 3.23's are too low for the 4.8...on the hwy it gets 19 - 20 and 14-15 city(2wd and 3.23's)... my 03 5.3 gets 15 in city and 18 on hwy (4x4 w/ 3.73's)

 

IMO, a 5.3 might be kindaok w/. a 3.23, if enuff mods help get it movin. Some even like 3.23 or thereabouts w/. a turbo, I just read that on here not long ago.
When I gave my above thoughts it was catered to a 5.3.

If I had a 4.8, I'd want at least a 3.42, preferably a 3.73, maybe even 4.10. Just a gut feeling. So when ya changing em?