The 4L80E shares 1st & 2nd ratios with the TH400. It's as follows:

2.48:1
1.48:1
1.00:1
0.75:1

With 35's, you'll basically lose your overdrive except maybe running down the Interstate at over 70MPH. But, the good news is that you can just snick it down in Drive with 4.10's for slower speeds & the motor shouldn't rev too bad. I know an awful lot of guys running 33-35" tires with 4.10's & no overdrive.

TEX

/wwwthreads_images/icons/wink.gif Got Mud?
G.U.M.B.O. Mud Racing

 

Hey TEX... You said you know an awful lot of guys running 33"s & 35"s w/ 4.10s. I'm wondering what their highway speeds/top speeds are and at how many RPMs. Also, what's their gas mileage like? I was thinking of going with 4.10s but a friend of mine told me I'd be happier with 3.73s and a cam. Currently have 3.42s. ;-(

Any thoughts on this?

 

Most of these guys are running older trucks, so gas mileage sucks & highway speeds are pretty much determined by how violently the truck shakes at speeds above 60./wwwthreads_images/icons/laugh.gif

RPM's can be determined with the following formula:

FD*MPH*336/TD = RPM

FD = Final Drive, which is the multiplication of the T-case gearing, the Tranny gear, & the axle gear. In high-range, the T-case ratio of any GM is going to be 1:1 & without overdrive, the top ratio of any tranny is 1:1. So, under those circumstances, FD would equal the axle ratio. If you have overdrive, you have to account for that. With a 5-speed, I believe O/D is .88:1. With the 4L80E it's .75, & with the 4L60E (aka TH700) it's .70:1.

TD is equal to your tire diameter.

On my '90 GMC, I have 3.73's & 265/75R16's. Those are basically 31.5" tall, so my highway RPM's are really low:

3.73*.70(TH700)*70MPH*336/31.5 = 1,949 RPM's.

That's really a little on the low side. Around 2,200 the truck runs better IMO.

We're talking about your Yukon, right? I doubt you'd get much use out of 3.73's. I don't care for mine with 265/75's. The only benefit I have with 3.73's is that I tow a trailer & I run in Drive when I do that. If I had 4.10's & ran Drive, it'd really make the motor rev harder. But, if I never towed I'd be wishing for 4.10's to use O/D a lot more than I do. And with bigger tires, I'd probably opt for some 4.56's. But, I have no idea what this'd do to fuel mileage. Actually, for my truck on stock tires, the ideal thing would be if I could keep the 3.73's & have an overdrive that wasn't quite so steep - like maybe .80 or .85:1. A gear like that would be a lot more useful.

TEX

/wwwthreads_images/icons/wink.gif Got Mud?
G.U.M.B.O. Mud Racing

 

Damn!!! You da man!!!

 

Haha, I think so too, or as they would say in northen Russia:
-He´s got his sh*t together maaan.

/Andreas

If a Blazer runs smooth, it isn´t really a Blazer/wwwthreads_images/icons/crazy.gif

 

Well that is what I came up with last night. I was crunching alot of numbers myself, like you explained in this thread. I figured that it would be good in drive and since where I live I take the interstate to work everyday and set the cruise for 75, my OD should be good.

'96 chevy 3/4 ton, 5.7, 14 bolt s/f 9.5", 4.10, lift coming soon

 

Andreas... We say that here in North America too!