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differential gearing / ratio help

kablooie

kablooie

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So I changed my diff oil this past weekend and found these numbers on the ring gear:
4020909 15 41 GM 12801

I couldn't see the pinion gear that well. So I don't know what it has. But my glove box sticker says truck came with 2.73 gears.

With my TH350 and 35" tires and the very high probability that I have 2.73 gears, I know I need to change them ASAP. Most, if not all of my driving is on the highway right now, but eventually I will be moving to Oklahoma and plan on being off road more.

BTW: I normally cruise on the highway around 65-70 MPH (real time) so therefore http://ratfactor.com/vehicle/gearratio.php?go=y&trans=th350&tcase=np208&diff=2.73&tire=35&crawler= says I am 1700-1850 RPM.

What is the recommended ratio to run?

ring.JPG

ring2.JPG

options.JPG
 
If you are going to stay at 35's then 3.73's or 4.10 would work well.

I assume this is a 10 bolt? You could probably find a set of axles (either two 10 bolts or a FF14 bolt and 8 lug 10 bolt front) that already have those gears for cheaper than swapping gears.
 
38377k5 said:
If you are going to stay at 35's then 3.73's or 4.10 would work well.

I assume this is a 10 bolt? You could probably find a set of axles (either two 10 bolts or a FF14 bolt and 8 lug 10 bolt front) that already have those gears for cheaper than swapping gears.
Click to expand...

This is a 12 bolt rear end, and I would rather keep it that way... seems simpler when I look at it that way and cheaper? (unless I am mistaken). Front is a 10 bolt.
 
kablooie said:
This is a 12 bolt rear end, and I would rather keep it that way... seems simpler when I look at it that way and cheaper? (unless I am mistaken). Front is a 10 bolt.
Click to expand...

12 bolt and 10 bolt are exactly the same as far as how simple each is.

If anything 10 bolt parts are cheaper because there are so many more of them.

Regearing your current axles (if you pay someone) will probably cost ~$1K. When you start breaking them you will wish you hadn't put that much money into them.
 
I agree with 38377k5

38377k5 suggested some 3.73 or 4.10, and I concur, especially if you're going to keep driving on the highway. I switched from 3.73 to 3.08 because I do all highway, with 29" tires, and a TH700 overdrive transmission, and my gas mileage went down by about thirty percent, and my highway speed at the same rpms' went up ten mph. Great Low end torque is purchased at the cost of poor high speed performance and excessive gas consumption. It's a trade off, a balance. So if you're going to continue to do mostly highway, probably the 3.73 would be best - if it's fifty-fifty with offroad or you're going to do towing in the Appalachians, then 4.10; if it's going to be 70% offroad or towing in the Rockies, then Slapperbar is right, something between 4.10 and 4.56 would be best. But at that higher ratio, you'll be spinning your wheels at stoplights with the slightest foot pressure on the gas pedal, even with 35" tires. And anytime your foot is on the gas pedal, you'll be able to watch the needle on your gas gauge go down.
 
user said:
38377k5 suggested some 3.73 or 4.10, and I concur, especially if you're going to keep driving on the highway. ... And anytime your foot is on the gas pedal, you'll be able to watch the needle on your gas gauge go down.
Click to expand...

Would 4.10s in a 700r4 be something to think about? 1850-1950 RPM at 65-70MPH but the low first gear is nice too.
 
I run a 700r4 , and have 4.56 . Its great in OD ( for me ) around 2200 at 65-70 , and a little high rpm ( for me ) in 3rd around 3000 rpm .
 
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