C6 and rear differential gearing

[Brian VT]

I'm sure this must have been discussed here but I guess my searching skills must suck.
Anyway, have many of you changed the rear diff. gearing to get lower RPMs at higher speeds? I don't drive much highway but we have a camp @ 1 hr. from here that I go to at least once a month and the speed limit on the turnpike is 70mph (most are doing 80mph).
What ratio seems to be the happy medium? Or is changing the gearing just not worth the trouble?

 

[IDIBRONCO]

I think that it would depend on your main use of the truck. If you drive in town a lot, pull something regularly, or just don't drive longer distances at highway speed very often, then I don't see where it would be all that beneficial to you to change out rear end gears for a once per month drive that's only one hour each way. It seems like a lot of work for that two hours each month. After these threads about swapping transmissions and rear end gears, you have to remember that our trucks were built during the days of the national speed limit of 55 MPH. They were at least adequate for that time. Today's trucks are more of a "one size fits all" design that our trucks can't match because they were never designed that way. With our trucks, it's a compromise to make it fit what you do most with it.

 

[u2slow]

3.54 is common and decent for highway. 3.07 and 3.31 are more rare. Beware you lose the grunt for hauling.

I can usually find a whole rear end in the ratio I want in the $100-400 range, depending what it is.

 

[Brian VT]

Thanks. I may keep my eyes open for a 3.54
I do mostly drive in town but I don't haul anything too heavy very often. Mostly my boat.
So it's easier to just swap the whole rear end than the gear(s)? What terms would I be searching for to find a whole rear end?

 

[Cubey]

Thanks. I may keep my eyes open for a 3.54
I do mostly drive in town but I don't haul anything too heavy very often. Mostly my boat.
So it's easier to just swap the whole rear end than the gear(s)? What terms would I be searching for to find a whole rear end?

It's a different set of challenges depending on which way you do it.

Changing the gears can be done with the truck on the ground with the wheels chocked, but you need to get clearances and shims right. IMO that's the harder option unless you really know what you're doing for a differential gear setup, or it'll eat itself up in short order.

Changing the whole axle means unbolting the shocks, the u-bolts for the leaf springs, the driveshaft, and the brake lines. It also means supporting the rear of the truck on stands while you're swapping the axle. If you have a flat paved driveway to work in, swapping the whole axle might be the better option. You should also do a rear brake job with new parts for the replacement axle while you're at it too since it'll probably need it.

 

[u2slow]

Www.car-part.com is a search engine for wrecking yard parts.

 

[Farmer Rock]

I believe your truck is 4x4, correct? If so, you have to change out the front end as well.
You may be better of just swapping out to an overdrive trans and leaving the gears be.


Rock

 

[IDIBRONCO]

Good point, Rock.

 

[Brian VT]

Makes sense. Thanks.
I guess I'll stick with what I've got since it seems solid and I don't think I want to go to an E40D.
If I start to really hate the C6 then I guess it'll be a ZF5 or maybe a Gear Vendor OD/UD.

 

[u2slow]

I got to borrow a buddy's F250 4x4 with IDI/ZF5/3.54 for almost a year. It's was a great truck for basic transportation and moderate chores.

 

[Noiseydiesel]

I believe, there are a few overdrive trans options available without going with the E4OD. If memory serves, which is getting more rare with each passing day, it was brought up on this forum not long ago and a search might turn it up. My '86 460 F250 standard cab is running a 3.79 and passes most everything on the road except a gas station.
Now if your thinking about changing gears on both front and rear, then the Truetrack for the rear and possibly an E-Locker on the front might be real slick unless all you desire to do is change out the gears.
Want more fun? Change out the diff's with another gear set and add the O/D.
Then again, change out the trans with an Allison 6 speed kit from CaConversions in Florida and get both a lower first and higher final gear.
Figuring my 4.10 F350 CC Dually and running 235/85-16 tires with an Allison,
80 MPH = 2169 RPM.
90 MPH = 2440 RPM.
100 MPH =2711 RPM.
110 MPH =2982 RPM.

Don't believe that brick will hit 110.
And you thought you would get an easy answer. . . . . .

 

[Farmer Rock]

Both my 6.9 and 7.3 are zf5s with 3.55s gears. Great combination imo.


Rock

 

[franklin2]

I believe, there are a few overdrive trans options available without going with the E4OD.

Zf 5 speed is the only other one that was stock. I highly recommend this swap. I had the c6 with 4.10's, and gained 3mpg just by going to the zf.

 

[u2slow]

I had a 84 diesel suburban once... I changed the 3spd auto w/4.10 to a manual 4spd w/3.21. I gained 5mpg.

 

[Brian VT]

That's cool.
I'm more concerned with noise and RPMs but I guess the better fuel mileage comes with it as a bonus.