Help deciding which rear gear to change to

[Frank M. Hardcastle II]

Thanks guys for all the help. This site is a wealth of information from very helpful and knowledgeable folks. I've been looking at Internet RPM/Speed calculators but still have trouble deciding which differential gear to use. I have a 1984 GMC C-3500 Crew cab Dually with a Banks turbo charged 6.2L and SM465 non-overdrive 4 speed with unsynchronized granny low 1st gear and reverse. I don't use 1st gear. The differential is 4.56:1 ratio. It appears that the closest I can be to the optimum 1800 RPM's at 75 MPH is 2220 RPM's with a 2.73:1 ratio differential gear. That sounds like it's just too high a gear ratio to me. With the 4.56 @ 65 MPH my RPM's are about 3600. I've thought about a 3.73 or 3.55 but was afraid that 3.55 would be too high. I only have use of 3 forward gears and primarily drive 1/2 in town and 1/2 on the highway. I very seldom pull anything but would like to retain the ability to do so if the need arises. In any case I don't think I'll ever pull more than an additional 10,000 - 15,000 lbs. Of course fuel consumption is as much an issue with me as anyone else. Please give me any advice you can. I'm going to have it changed out the end of this coming week or the beginning of the next. Thanks again for all your help.
Frank

[john8662]

Given that you're geared 4.56 I feel that a change to 3.73's would work out really well.

Still have good pulling ability and better RPM's down the highway.

J

[RoverIIa]

Don't forget that you can change tire sizes to sort of fill in those "in between" ratios. It is expensive on a dually, but can be done.

I run a 83 6.2 with SM465 in a 4x4 with 3.42 Gears and 33x10.5 tires. It gets great mileage at California freeway speeds (sometimes as much as 22-24 when the winds are right) and I only use the LOW gear when I tow something...my trailer grosses out at 7k, and I try not to overload it, so I'm not towing much.

I'm in the process of adding a turbo now. I am excited to see how that changes the equation. Ideally, no more downshifting on steep grades.

[JeepSJ]

You also have the option of pulling out that SM465 and swapping in a NV4500. Having the overdrive will help significantly. Then you could drop to something like a 4.10 which will get the RPM down but still give good towing performance.

[Frank M. Hardcastle II]

[QUOTE=john8662]Given that you're geared 4.56 I feel that a change to 3.73's would work out really well.

Still have good pulling ability and better RPM's down the highway.

To get the RPM's down it looks like I'll need to be in the 2.73-3.08 area. Sounds high to me. Do you know how that will effect my performance?

[Frank M. Hardcastle II]

You also have the option of pulling out that SM465 and swapping in a NV4500. Having the overdrive will help significantly. Then you could drop to something like a 4.10 which will get the RPM down but still give good towing performance.

I would love to go with an NV4500 however I can't afford to do that now. The gear change is $650 and within my budget.
Thanks
Frank

[Frank M. Hardcastle II]

Don't forget that you can change tire sizes to sort of fill in those "in between" ratios. It is expensive on a dually, but can be done.

I run a 83 6.2 with SM465 in a 4x4 with 3.42 Gears and 33x10.5 tires. It gets great mileage at California freeway speeds (sometimes as much as 22-24 when the winds are right) and I only use the LOW gear when I tow something...my trailer grosses out at 7k, and I try not to overload it, so I'm not towing much.

I'm in the process of adding a turbo now. I am excited to see how that changes the equation. Ideally, no more downshifting on steep grades.


The tires I have now are stock. I think they're something like 265-285/85/16. Seems like I measured them at 31 inches.

[Warren96]

You may not be able to put a much smaller gear number than a 3.73 into the housing that your truck has.Check with a place like DrivetrainSpecilists.com or how about RingPinion.com The price of changing a gear is one thing,but changing the housing.....

[john8662]

Check this out...

With 31" tires:

1:1 Trans Drive ratio (no overdrive)

3.42 ratio

65 MPH: 2410
70 MPH: 2596

3.73 ratio

65 MPH: 2629
70 MPH: 2831



Now, with the stock size tires that come on a C3500 pickup.

225/75 R16's

1:1 Trans final drive ratio

3.42 ratio

65 MPH: 2851
70 MPH: 2748

3.73 ratio

65 MPH: 2783
70 MPH: 2997

I figured in 70 MPH into these equations because I figured that was a more realistic speed for more traveling conditions.

If a person wasn't going to pull much (if anything) and has a Banks Turbo on their 6.2 (more power) then a 3.42 gear ratio could possibly work to increase fuel mileage. For all all-around good ratio, the 3.73 works well, nice for pulling, yet manageable for fuel economy.

But, as usual, an OD tranny would help.

If the budget is just for gears, it still makes sence for the fuel savings.

Consider a few angles tho...

Will you ever convert to an OD tranny? If so, I don't think the 3.42's will serve as well as the 3.73's will in OD.

For non-OD, I vote 3.42, if, as mentioned, your housing holds them.

You should have a GM 14 Bolt Full Floating differential 10.5" ring gear axle.

J

[AJMBLAZER]

He's got a Dana 70 rear axle. For some reason GM used Dana 70's under the dually pickup trucks of that era. SRW's and chassis cab trucks all got 14FF's. Interesting side note-if you swap SRW hubs onto a chassis cab 14FF it's about the perfect width for a Jeep. Anyways...


You might look around for a complete axle out of a gas powered diesel. I'd bet there's some guy out there with a 4x4 dually who wants 4.56 gears and has some nice high 3.xx gears. I've seen SRW 14FF's go for $250 or less and the dually D70's aren't as desirable because they only work with duallies.

www.car-part.com is often very helpful.

[DmaxMaverick]

....You might look around for a complete axle out of a gas powered diesel....

Lost me with that one. In my experience, those gas powered Diesels don't run well enough to need an axle.

[AJMBLAZER]

Derp...gas powered dually...:rolleyes:

[Frank M. Hardcastle II]

I think Im going with a 3.55 on Monday.
Thanks for all the help.
Frank

[Frank M. Hardcastle II]

OOPS! Looks like I made a mistake. The tires are 235/85/R16's. Do you think I'll be OK with a 3.55?

[john8662]

235/85 R16's with 3.55 gear ratio will grant you:

2445 RPM @65 MPH
2633 RPM @70 MPH

J

[oilburnertoo]

I had a '91 P/U with a 6.2 and SM465 & 3:73 rear , 235.85.16 tires. I swaped to a NV4500 trans and gained a solid 6-7 mpg increase. I couldn't believe it made that much difference but it did. I still had all the power I needed. FWIW keep the trans swap in mind as at todays fuel prices it wont take long for a payback from such an investment.

[Ebrown]

My blazer came with 3:43 and 31 tire,I have a 700r4. I'm swaping axles to 3:73 for towing and the change in the tire size I made I run 31x12.50x15 for the winter and 33 boggers or 34 swamper LTB for the summer. I would look around your town for the dana 70 rear before you spend the cash to swap out gears any / most gasers would cut off a left arm to get there hand on it d 70 with 4:56 gears

also: Petersens 4wheel & Offroad May 2007 pg. 68

Just swapped out a 400 trans for the NV 4500 and 6.2 TD

[Frank M. Hardcastle II]

My blazer came with 3:43 and 31 tire,I have a 700r4. I'm swaping axles to 3:73 for towing and the change in the tire size I made I run 31x12.50x15 for the winter and 33 boggers or 34 swamper LTB for the summer. I would look around your town for the dana 70 rear before you spend the cash to swap out gears any / most gasers would cut off a left arm to get there hand on it d 70 with 4:56 gears

also: Petersens 4wheel & Offroad May 2007 pg. 68

Just swapped out a 400 trans for the NV 4500 and 6.2 TD

Thanks for all the help guys. Changed out the 4.56 for a 3.55 today. Haven't had the opportunity to put enough miles on it yet to check the MPG but things sure do feel better. Driving to the gear shop this morning:

84 C3500 6.2L Banks Turbo
SM465
4.56 Ratio
65 MPH
3000-3200 RPM's
250 degrees

Coming home this afternoon:

84 C3500 6.2L Banks Turbo
SM465
3.55 Ratio
70 MPH
2500-2700 RPM's
210 degrees
$900.00 less in my pocket.

The only thing I have noticed is a slight shudder taking off in 1st gear from a stop. Some might consider my 1st gear to be 2nd as 1st is a low unsincronized granny gear. That shudder, and the loss of quick accelleration that I expected, are the only differences so far. I probably should have gone with the 3.73 as suggested by several however my ignorance and determination won out in the end. Sure hope my mileage goes up considerably. I don't think I can afford to have it changed out again down to the 3.73 any time in the near future if this shudder becomes more pronounced. I kept the gears that were changed out so if anyone would like to make a change and see what it's like to run at redline regularly let me know. I have just what you need.

Thanks again,
Frank

[Ebrown]

Frank,
The shudder is more and likely in the clutch, I would drive it for a while and see what happends. My 1986 ss w/ build 350 and I get the same thing in between 1st and 2nd gear shudders alittle. Post the Gears here or www.ck5.com or your local 4x4 clubs

[Frank M. Hardcastle II]

Frank,
The shudder is more and likely in the clutch, I would drive it for a while and see what happends. My 1986 ss w/ build 350 and I get the same thing in between 1st and 2nd gear shudders alittle. Post the Gears here or www.ck5.com or your local 4x4 clubs

Just had the clutch replaced less than 100 miles ago. Mechanic told me he really wanted my 12 inch plate? and was really tempted to switch it with a 10 inch for his own vehicle. This bothers me and now I wonder if he did take it. If I have to worry about stuff like this I don't think I'll use him again. Can that even be done? switching out a 10 inch for a 12 inch ?

[Ebrown]

I'm not sure if the can be swapped out from 12 to 10in disks.

[john8662]

whats up with the 250 then 210 degrees, I hope you're not talking about coolant temp here.

Glad that the RPM's are down now, make a huge difference for sure.

I think you'll like the 3.55's after some time.

J

[Frank M. Hardcastle II]

whats up with the 250 then 210 degrees, I hope you're not talking about coolant temp here.

Glad that the RPM's are down now, make a huge difference for sure.

I think you'll like the 3.55's after some time.

J
Yes coolant temperatures. Is this a problem?

[john8662]

250 degress is on the bleeding edge of overheating.

Good that slowing the engine down quite a bit has helped the temps stay down.

Running an engine @250 degrees for extended amounts of time is dangerous to it's health.

I do understand the issue with hight RPM and a Banks Turbo on a 6.2L, but usually when pulling.

what thermostat are you running?

I'd suggest a 180 degree Robershaw stat to help keep things in check and also verifying that the fan clutch is working properly, as well as the condition of the radiator.

You still need a little more safety margin on your temps.

Keep it just under 200 is preferable under most circumstances.

Mine all run at the First temp mark unless pulling, they'll run around 200.

J

[Frank M. Hardcastle II]

250 degress is on the bleeding edge of overheating.

Good that slowing the engine down quite a bit has helped the temps stay down.

Running an engine @250 degrees for extended amounts of time is dangerous to it's health.

I do understand the issue with hight RPM and a Banks Turbo on a 6.2L, but usually when pulling.

what thermostat are you running?

I'd suggest a 180 degree Robershaw stat to help keep things in check and also verifying that the fan clutch is working properly, as well as the condition of the radiator.

You still need a little more safety margin on your temps.

Keep it just under 200 is preferable under most circumstances.

Mine all run at the First temp mark unless pulling, they'll run around 200.

J

When I bought it in South Dakota in December it had a 160 in it. Things were cold up there. People were actually parking their trucks on frozen ponds. When I got home I couldn't get it to heat up enough to make the heater work properly. I changed it out to either a 180 or 185. I can't remember which. It was the only one available and was about 5-10 degrees off of what GM said I should use. Now that things are warming up a bit 70-75 degrees I'm seeing higher temps. Seems like it now runs between 195 - 210, maybe 215. I have seen it above 250 without any indication of overheating like steam, loss of power, smells or loss of fluid.