4.10's

 

Here's 2 good resources, to use together, to understand the gearing:

MOTOR - A Rear Axle Calculator

2011 Chevy Silverado 1500 4.10 or 4.56 Gears?

I think since you drive 80% highway... you'll be disliking the 3.73s with 31.5" tires. ~1900rpms @75mph is gonna be ~14-15mpg... that's a joke on the highway.

 

I like to use Randy's ring and pinion calculator:
Calculators
And I've actually been emailing back and forth with the guy who made the other website you linked to. He has been very helpful.

For the rest of your comments, I'm not sure I understand what you were hinting at. First, I said that I drive 80% city, not highway and I have a 32" tire, which would put me at 2000RPM at 75mph with 3.73s. Second, are you suggesting that 1900rpm is high or low at 75mph? I was shooting for a 2000-2100 RPM at 75mph.
I'm always interested in hearing what people consider an "ideal" highway RPM. It's suprising to see the wide range of opinions on this.

 

Id say 1900 is low. My truck was around that at 75 and down shifted like crazy! Then again im lifted with 34' tires.4.56s rocked my trucks world!

 

Meet in the middle...Motive 3.90s

 

Actually I would love to do that! Only problem is that I don't think it's available for the 8.25IFS front differential.

 

That's what I get for not reading thoroughly. :doh:

Nope, 3.90s are not offered by anyone for the IFS 8.25". Bummer! That said, I would go with a set of Richmond 4.10s. I don't think you will regret it. The difference in hwy rpm will be minimal and shouldn't effect your fuel economy any worse than a 3.73.

 

So the only options are 3.08, 3.42, 3.73, 4.10, 4.56, 4.88?

nothing between 3.42 and 3.73?

 

I have stock 3.73's in my truck and I'm torn between 4.10's and 4.30's for my dd lol. Go 4.10's at least.

 

4.10s with 32 in tire equals 2100 at 70. 2450 at 75. When I had 3.23s with same tire it was 1700 rpm and it always downshifted if the road wasnt flat. Milage increased with 4.10s to 18 mpgs from 16 and thats still not being nice to it. If its 4x4 its a little heavier and with 4.10s it should help around town. I think the higher rpm put it closer to the torque curve and now my truck never downshifts. Mines 4l60 but being that the OD ratio is .03 difference between 6l80 it doesnt matter.