C30 dually rear diff gearing options help please

[evan love]

Hello,

I just bought a 1979 C30 single cab dually with a very recently rebuilt 454 (100 miles ago) and I assume th400. It is also the camper special 10,000 GVW Silverado with 70,000 original miles.

I plan to tow a car trailer with a '73 K10 on it, so I want to retain that level of towing, but also would like to get better gas mileage and higher highway speed.

So, the question is, what is the absolute least expensive way to get better gas mileage and lower rpms to raise highway speed given that everything is in good condition and I don't really want to work on this one forever, I'd rather drive it.

It does have 4.56 gears in the rear diff, so perhaps that is the place to start.

Gear Vendors looks super cool, and easy, but $3,500 would take a while to make back up in improved gas mileage, wouldn't it... ...anybody done the math on such things?

I figure maybe 5,000 miles per year, which doesn't justify spending too much on mpg saving hardware, does it?

thanks, I appreciate input on this, and I don't actually know what mileage it gets now

[evan love]

pics

[evan love]

for the good people who already replied to my other post, please forgive this similar one, I thought the other one had failed to post...

... and, feel free to add anything more you'd like to share here or there

thanks,

evan

[Big Chip]

I have 3.73's in mine and she's a screamer at today's highway speeds. I have a 4L80 for mine that I haven't installed yet but I'm hoping that helps somewhat. Other than that I'd say GV would be the way to go.

[Big Moose]

I had 4.56s and went to 4.10s.when the bearings went kaput. didn't make a big difference in fuel mileage, maybe 0.5 mpg highway. I have a 6L LS/4L80 in it. it originally had a 350/th350 and it was screaming at highway speeds.

[Honest Bob]

With a 454 I'd be tempted to go 3.73 but I dont have any real world experience with that combo. 4.56 is really low and pretty popular with the 4x4 crowd you might be able to trade gear sets for nearly free.

Using the gearing calculator at http://www.grimmjeeper.com/gears.html it looks like 3.73's would drop almost 600 rpm's at 70mph compared to 4.56's.

[Cody75]

I have the 454 and 3.21 rear gear with the SM465 manual transmission in my 1984 C2500 from the factory. It does decent on the highway (about 13 mpg) even with that non-overdrive 4th gear. Difference being though is the transmission has a granny gear so it can get away with having a taller (lower numerical) rear gear and still tow just fine.

Had a 1975 with a 350 and TH350 transmission and 4.10 rear gears that got worse mileage at about 11 mpg highway at about 70-75ish mph. This led me to swap the transmission for a 700R4. That overdrive gear brought my highway mileage up to 14-15 mpg.

My suggestion would be to find a 4L80 and swap that in. Similar basic construction to the TH400 with the planetary gearset, but with the addition of an overdrive gear.

If you can get away with it cost wise, drop that rear gear down to a 4.10 or 3.73 as well and you will see an increase in fuel mileage.

[kwmech]

If you plan on keeping the truck long term then suck it up and put in either a Gear Vendor or a 4l80e. I would do this first. If that is still not enough then step up to a 3.73. If the engine is somewhat built then it will have no problem pulling any weight through the taller gear.

[evanlove]

Yes, I like the truck a lot, so I plan to keep it and would love to have it drive well while towing and while not towing.

Unless I'm missing something, I feel like a 4l80e could actually be cheaper than a gear vendors, and have a few advantages over the gear vendors (i don't have to flip a switch to shift to overdrive, lock up torque converter, more of a percent overdrive compared to gear vendors unit).

The 454 is newly rebuilt but basically stock. Currently has headers and an Edelbrock carb, but might need to put it all back to stock to pass California emissions.

Hmm, I wonder if California emissions cares what transmission is in it?



Maybe the answer is like you say, 4l80e to start, then maybe diff gears if it needs still lower rpm's for highway driving and can give a little without losing towing power.

Any thoughts on how many rpm's is too many in a stock 454 in good shape for long highway hours?

Thanks,

I appreciate you sharing

[evan love]

I just found this document from

https://www.gmheritagecenter.com/doc...olet-Truck.pdf

and apparently the stock setup from Chevy in 1979 for the 454 dually was a 3.73 rear diff, with 4.10 and 4.56 being options...

... and that is with the Camper Special 10,000 GVW...

... with a non Camper Special they came stock with 3.21 diff gear ratio.



I guess they thought it could still handle to 10,000 GVW total weight with the 3.73...

... though that is more about weight in the bed than pulling a trailer.



Any thoughts on the difference in carrying 2 tons verses towing 3 or 4 or more tons with regards to diff gears or overdrive transmissions?

Thanks

[THI]

My 1979 C3500 Dually has the 4.10 gears (also a Camper Special) and 454 - I get about 10.5 on the highway with the TH400. My 1978 C20 (Camper Special) HAS THE 454 and 3.73 gears - I get around 11.5 highway with that combo - both empty. Load up the C3500 and it only drops to 10 - hard to say what the 1978 C20 does loaded as it is time to rebuild that 454 (it drops to around 8 when loaded had has no power!).

The 1979 C3500 is a 58,000 mile original lightly used in it's previous life. The 1978 is a 87,000 mile original that spent it's original life as a tow vehicle for a local RV dealership that worked it hard all summer and parked it during the winters.

My 1997 K2500 Suburban gets 16 mpg on the road empty and will get about 12-13 when towing a 7,000# trailer - 4L80E with 3.73 gears. If you are going use the truck regularly, I would recommend the 4L80E combo with the 3.73 gears - I am keeping my other trucks with their TH400's because they are now Limited Use Classics - the 1978 will eventually get a full restoration.

[evan love]

Great info THI,

How fast can you drive the 1979 with the 4 10 gears on the highway, and how many rpm's is that?

Thanks

[THI]

4.10's are just under 3,000 @ 60 mph - 3.73 are around 2,700 @ 60 mph

It will go faster than that, but I like to limit myself to 65 mph when towing, have gone as much as 75 mph on interstate, just felt I had better control @ 65

[geezer#99]

Your tire height effects your gearing.
What tires are you guys running?
What size?

[evan love]

Good question on tires size geezer,

I was wondering that too.

I don't know what tire size i have, I bought the C30 dually in Colorado from my dad's neighbor the day before I flew back to California, so I'll have to ask him to go outside and look at the tires for me.

I'm trying to figure out what all I'll need to do to drive the dually, hopefully towing a K10 shortbed, back to California in the spring.

What size tires do you have THI?

Obviously dually tires can only be so wide before they hit each other, does anyone know how big or tall a dually tire can be with stock rims?

Another question, how many rpm's is comfortable for a stock 454 to run all day on the highway? Carbureted original 1979 type 454.

Thanks

[72c20customcamper]

Cheapest would be gear change. But at maybe 1 and 1.5 gallon increase in milage and driving you say about 5k miles a year it's going to take a long time to recoup. My 85 k20 has a 700r4 and 3.21 gears . I'm looking to put either lower gears or a 400 in it . I dont use od because if I hit an incline even at highway speeds the trans hunts. I get almost the same mpg in od as I do in 3rd. I had it looked at by my trans guy and he said nothing wrong with the trans and it looks to have all the upgrades .

[kwmech]

As far as tires, I had 255/85r-16s on my tow truck. They come in at 33'' tall. Sidewalls did not touch but it was close (about 1/2''). BUT with your 2wd there may be an issue on the front with them rubbing the fenders on a turn. My wrecker was 4x. I see on the spid your truck came with 7.50/16s. A comparable size for those is a 235/85r-16 which will be 31 (ish) in height. some of the duallys came with 215s which really looks stupid.

[THI]

I am running LT245/75R16E on my 78 C20 and LT215/85R16E on my 79 C3500 (GMC)

The 78 originally came with 9.50-16.5 which are very difficult to find these days, I got a set of rims off a 1986 K30 which came with LT235/85R16. The LT245/75R16 is very close to the same diameter as the original 9.50-16.5 as is the 215/85R16 on the Dually.

On a Dually I would recommend either the LT215/85R16E or the slightly larger LT235/85R16E

[evan love]

I guess the 235's sound good.

For those with a relatively stock 454, If I didn't mind paying for the gas, and didn't mind the louder noise, how many RPM's is comfortable for the engine to do all day on the highway.

I just don't want to send the engine to an early grave. I'd like to take good care of it, so how many RPM's could I do all day on the highway without doing any damage, and with the engine being happy about it all?

thanks