C6 and rear differential gearing

u2slow said:

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.

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Farmer Rock said:

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


Rock

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I completely agree with both of these.

Brian VT said:

I'm more concerned with noise and RPMs

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These engines can be run against the governor for hours at a time so don't worry too much about high RPM. The fuel mileage will suffer from doing that, but the engine won't be hurt. Unless your cooling system isn't up to par and the sustained high RPM causes it to overheat. I think that your cooling system would have to be in pretty bad shape or maybe the fan clutch would need to be replaced if it got hot while running empty at any RPM.
As far as the noise goes, one down side to the IDI engine is that it's noisy. That's why I have a loud (to me anyway) radio in my truck.

Just think of how much less engine wear you have by dropping the rpms.

franklin2 said:

Just think of how much less engine wear you have by dropping the rpms.

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Keeping your foot out of it helps too. This month, I got a whopping 9.7mpg from Brownfield TX to Yuma, AZ in my motorhome by staying at 55mph max. That's with the winter blend fuel too. That's with a C6 and 4.10 rear end pulling a 1956 fiberglass box trailer.

That's pretty impressive to me. Especially where you were at when you bought the RV.

IDIBRONCO said:

That's pretty impressive to me. Especially where you were at when you bought the RV.

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That first summer when I had it, it was getting 9'ish with a good bit of headwinds (no trailer). Then the mechanical pump began weeping to the ground quite a lot (around the time I stopped by there). Then I put in the electric pump but covid hit and I didn't drive much after that in 2020, except to get out here in Nov 2020. It got timing done before that in TX after the epump. But it as headwindy a lot and I was in a hurry that was probably why MPG was crap last time. But also maybe the IP was gummy from sitting so much in 2020. It got driven a fair bit in 2021 and was getting 8.5-9'ish.

If it didn't cost ~$3400 (new anyway) for a GV, plus a shorter driveshaft, I'd consider getting one to help MPG more. I keep my eyes pealed on CL and FB but even used is $2k usually.

Brian VT said:

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

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Changing the gear ratio say from 4:10 to 3:55 will not make the engine louder.
You will have lower RPM`s with the 3:55, but a higher rear end gear ratio.

With a 4:10, the engine rotates 4:10 times for one revolution of the rear wheels.
With a 3:55, the engine rotates 3:55 times for one
revolution of the rear wheels.

My 86 6.9 has the T-19 4-SPD trans and a 3:55 Diff,
I can easily get 20 MPG with it empty.
Throw on the car trailer, and all bets are off. Also no Turbo, it is naturally exasperated.

I have hauled lots of loads with the trailer and in the bed. Only draw back is being on a steep grade, and starting from a stop, have to raise the RPM`s and play with the Clutch to get her moving.

A Gear Vendor would be great, it would give you that OD Gear, plus you can split the gears shifting.
I pulled one from PNP last month that was under a 89 F-250 7.3. They had no idea what it was, thought it was a Transfer Case. Got it for $160, but had to go back for the Tail Shaft that connected it to the E4OD, and that was $34.

Here is one in Santa Cruz Ca. CL

One in northern Ca,


There was one I was watching in the Seattle, Wa. area with a T-19 and the GV attached to it for $1475. Looks like it got sold or listed on E-Bay

Yeah they are pricy new.

Goat

Old Goat said:

Also no Turbo, it is naturally exasperated.

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Shucks, even with a turbo, mine gets exasperated sometimes.

I re-geared my 89 ZF5 from 4.10 to 3.55 and gained 2.5 mpg, lower RPM and a quieter cab. With a C6 I would re-gear to 3.31, and use the low range in the T-case if you need to get a heavy load moving. Due to the cost of doing both axles a trans swap might be cheaper. If you were closer than 2500 miles I would tell you that I have a 3.55 axle set for $250/pair and an E4OD trans and T-case for $350/set.
If memory serves; 3.55 and a C6 will get you 2000 RPM at 65mph. What are your gears now?

The tach. doesn't work but judging by the sound of the engine at 60mph I would assume it's got 4.10

Check your door tag for the axle code. You may have 3.55s but without overdrive it will still be at higher rpms. My 87 was a 4 speed with 3.55s, and was pretty loud on the highway, but now with the zf5 swapped in (overdrive) it is a lot nicer, and better fuel mileage of course.
3.55 gears seem to have been very common with ç6 transmissions, and the 4 speed due to lack of overdrive. The 88 ç6 I used to have had 3.55s and basically drove the same as my 4 speed.



Rock

Just checked. It's C5 (4.10)