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Old 01-29-2018, 09:22 PM   #1
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Need some help with differential math

So.....I've decided to swap my differential gearing....it's currently a 4.78

Engine is DT466E, 175HP@2300rpm, 430ft-lbs@1500 rpm (had the road speed limiter removed Friday).

Will be putting 11Rs (41") on the rears.

My Spicer ES43-5D 5 speed manual ratios are as follows....1st 7.25, 2nd 3.98, 3rd 2.23, 4th 1.29, 5th 1.00

1st and 2nd are next to worthless.....I can only roll about 10 feet in first b/4 having to shift, and 2nd is about another 30 feet.

Ideally, I'd like to cruise at 62-65mph at 1600-1800rpm, and have 72-75mph available for passing/merging.

Am I right in thinking that I'll need 3.73s or 3.91s to get me where I want to be? And is there a formula to tell me top speed in each gear at a given rpm?

I'd appreciate any help to shed some light on this for me.


Don
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Old 01-29-2018, 09:28 PM   #2
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Quote:
Originally Posted by plfking View Post
So.....I've decided to swap my differential gearing....it's currently a 4.78

Engine is DT466E, 175HP@2300rpm, 430ft-lbs@1500 rpm (had the road speed limiter removed Friday).

Will be putting 11Rs (41") on the rears.

My Spicer ES43-5D 5 speed manual ratios are as follows....1st 7.25, 2nd 3.98, 3rd 2.23, 4th 1.29, 5th 1.00

1st and 2nd are next to worthless.....I can only roll about 10 feet in first b/4 having to shift, and 2nd is about another 30 feet.

Ideally, I'd like to cruise at 62-65mph at 1600-1800rpm, and have 72-75mph available for passing/merging.

Am I right in thinking that I'll need 3.73s or 3.91s to get me where I want to be? And is there a formula to tell me top speed in each gear at a given rpm?

I'd appreciate any help to shed some light on this for me.


Don
Plug your numbers in here and play to your heart's content

Gear Ratio Calculator

That page has a tire diameter calculator at the bottom too.
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Old 01-29-2018, 09:39 PM   #3
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Quote:
Originally Posted by plfking View Post
So.....I've decided to swap my differential gearing....it's currently a 4.78

Engine is DT466E, 175HP@2300rpm, 430ft-lbs@1500 rpm (had the road speed limiter removed Friday).

Will be putting 11Rs (41") on the rears.

My Spicer ES43-5D 5 speed manual ratios are as follows....1st 7.25, 2nd 3.98, 3rd 2.23, 4th 1.29, 5th 1.00

1st and 2nd are next to worthless.....I can only roll about 10 feet in first b/4 having to shift, and 2nd is about another 30 feet.

Ideally, I'd like to cruise at 62-65mph at 1600-1800rpm, and have 72-75mph available for passing/merging.

Am I right in thinking that I'll need 3.73s or 3.91s to get me where I want to be? And is there a formula to tell me top speed in each gear at a given rpm?

I'd appreciate any help to shed some light on this for me.


Don
A 4.78 will only have you doing around 65 at redline with 11R's.
4.10 would be about PERFECT for you. It would give you 76-77 at redline and 60 mph at 2000 rpm.
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Old 01-30-2018, 12:57 AM   #4
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ohhh, whew. My heart missed a beat- I thought you needed diff eq help.

dif.jpg


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Old 01-30-2018, 02:16 AM   #5
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4.10 or 3.73 either is a viable option. and will make that 1st gear a lot more usable.
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Old 01-30-2018, 02:19 AM   #6
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ohhh, whew. My heart missed a beat- I thought you needed diff eq help.

Attachment 18744


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yikes horrors of college just flashed back through my head.
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Old 01-30-2018, 08:47 PM   #7
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Thank you, guys, for your input and the calculator.

Food for thought.


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Old 01-30-2018, 09:24 PM   #8
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Do you plan on towing anything?
Gearing for road speed reduces your Towing ability to even pull a car on a bar.
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Old 01-30-2018, 11:50 PM   #9
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MAY occasionally tow a 6x8 trailer with a couple of dirt bikes, but no plans for anything heavier.

Even if 5th gear becomes nothing but a flat-country OD gear, that would be okay with me.


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Old 01-31-2018, 07:40 AM   #10
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my 6th gear on my redbyrd is nothiung but a flat country gear.. has me Taching about 1550-1600 at 65 MPH.. id like to have a little lower gearing than that .. omnce I go up the least little incline I drop back to 5th which has decent power. but you may want to keep your RPMs in that 1800-1900 range in top gear at 65 or your planned cruising speeds.. seems the engines are happier there.. at least my 444E is.
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Old 01-31-2018, 08:36 AM   #11
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3.90 would be good
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Old 01-31-2018, 08:53 AM   #12
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3.90 would be good
thats probably what i need in my bus
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Old 01-31-2018, 12:25 PM   #13
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Do folks usually swap the entire rear end or do they replace the internals of their current diff?
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Old 01-31-2018, 12:41 PM   #14
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Do folks usually swap the entire rear end or do they replace the internals of their current diff?
Usually it is far cheaper and easier just to swap out the "chunk" (carrier housing). The only reasons I would swap out an entire axle is:

1. The old one is damaged beyond reasonable repair (as in, I could replace it cheaper).
2. Swapping spring ride for air ride (and the old one won't work).
3. Needing a heavier duty axle.
4. Parts for the old one are no longer available.
5. Swapping Budd/Dayton style rims. This would usually include doing the same on the front axle as well.
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Old 06-13-2018, 06:03 PM   #15
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update?

Hey was wondering if you actually swapped the rear diff out, I have a 96 3800 with a DT 466 mechanical rated at 190HP with the 4.78 rear and was wondering what your speeds and RPMs are, I have a 6+1 spicer tranny. I want to go to a 4.10 as well but would like to hear about yours first
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Old 06-13-2018, 09:56 PM   #16
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Sent you a PM
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