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Old 10-12-2018, 03:41 PM   #1
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Rear end ratio with Allison 2500

Hi All,


I discovered my new 10 window bus has a 6.17 rear end ratio and based on the 11r22.5 tires I calculated my engine speed to be 2130rpm = 60mph.


In looking at the engine torque/HP curves it seems to me that 1600-1800rpm would be the best freeway RPM for my 210HP DT466E.


My question to the group is "What rear ends seem to work well for 8-12 window sized buses with a A2500 transmission?"



I will be buying a used 3rd member and my initial guess would be a 4.66 to 1 (5th:1700rpm=63mph, 1st gear:1700rpm=13mph) but I am using that as just a starting point for discussion.



I would like to avoid changing the carrier twice.
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Old 10-12-2018, 05:25 PM   #2
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Quote:
Originally Posted by PeteinSF View Post
Hi All,


I discovered my new 10 window bus has a 6.17 rear end ratio and based on the 11r22.5 tires I calculated my engine speed to be 2130rpm = 60mph.


In looking at the engine torque/HP curves it seems to me that 1600-1800rpm would be the best freeway RPM for my 210HP DT466E.


My question to the group is "What rear ends seem to work well for 8-12 window sized buses with a A2500 transmission?"



I will be buying a used 3rd member and my initial guess would be a 4.66 to 1 (5th:1700rpm=63mph, 1st gear:1700rpm=13mph) but I am using that as just a starting point for discussion.



I would like to avoid changing the carrier twice.
4.66 gets you there. But it's not the carrier that you change out, it's the Ring and Pinion gears you change. It's possible that you might have to change the carrier when dropping from the 6.17 down to a 4.66, but not likely. In either case, the carrier has to come out when you change gears. The ring gear is attached to the carrier and the pinion gear is attached to the differential housing/drive shaft - both internal to the differential.

If and when I decide to gear down, I will replace the carrier with an LS carrier - better control and less chances of getting stuck with power to both sides.
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Old 10-12-2018, 06:09 PM   #3
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By carrier I mean a complete 3rd member (like a Ford 8 inch)
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Old 10-12-2018, 06:57 PM   #4
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peteinSF

I did what you are doing.... 4.88 rear gears in mine, but I went to 3.73 cost about $800 in my case. calculated for me 3.73 gears, 245 70 19.5 tires at 65 mph will put me at 1875 rpm... I think I used michelin data that has the tire I want to use doing 622 revolutions per mile. so at 65 mph I am at some thing like 674 to 677 revolutions per minute at 65 miles an hour and I have an over drive of around .73 In the end works out to around 1875 in 6th gear and 2525rpm in 5th gear at 65 mph

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Old 10-20-2018, 12:48 PM   #5
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My survey is clearly a flop...


I was hoping to see what range of axles were attached to 2500s out there.


The good news is that used Spicer 3rd members are generally < $500 so I guess I don't need to get it perfectly right.
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Old 10-20-2018, 12:50 PM   #6
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Quote:
Originally Posted by PeteinSF View Post
Hi All,


I discovered my new 10 window bus has a 6.17 rear end ratio and based on the 11r22.5 tires I calculated my engine speed to be 2130rpm = 60mph.


In looking at the engine torque/HP curves it seems to me that 1600-1800rpm would be the best freeway RPM for my 210HP DT466E.


My question to the group is "What rear ends seem to work well for 8-12 window sized buses with a A2500 transmission?"



I will be buying a used 3rd member and my initial guess would be a 4.66 to 1 (5th:1700rpm=63mph, 1st gear:1700rpm=13mph) but I am using that as just a starting point for discussion.



I would like to avoid changing the carrier twice.
5.29 is nice with that setup and full size tires.
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Old 10-20-2018, 01:29 PM   #7
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Depending on what rear axle you have in your bus, you will find a bunch of carriers being sold with ratios in the high 4's (4.90, 4.73) from buses and trucks with direct drive transmissions,also 4.44 seems to be real common a couple of years ago when I was looking
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Old 10-20-2018, 08:41 PM   #8
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Best bet is to run all the numbers through a gear ratio calculator...like the one below.



Gotta know desired highway RPM, tire diameter, trans final drive ratio, etc., but it will tell you just where you need to be on the rear end.


Engine RPM Calculator


(the one at the bottom of the page is best for your calcs).
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Old 10-21-2018, 12:44 PM   #9
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Quote:
Originally Posted by PeteinSF View Post
Hi All,

I discovered my new 10 window bus has a 6.17 rear end ratio and based on the 11r22.5 tires I calculated my engine speed to be 2130rpm = 60mph.

In looking at the engine torque/HP curves it seems to me that 1600-1800rpm would be the best freeway RPM for my 210HP DT466E.

My question to the group is "What rear ends seem to work well for 8-12 window sized buses with a A2500 transmission?"

I will be buying a used 3rd member and my initial guess would be a 4.66 to 1 (5th:1700rpm=63mph, 1st gear:1700rpm=13mph) but I am using that as just a starting point for discussion.

I would like to avoid changing the carrier twice.
Gearing up to a higher ratio(lower number ) and getting it exactly right is not easy the first time.

Getting the RPM you want is So very easy. pick the gear, speed, and tire height and you have it.

you sound close on that 4.66 gear to me.

What RPM does that trans shift at WOT?
What is your governed speed now?

But a school bus is not a F250 pickup truck. The weight and Aerodynamics are NOT even close. Sometimes the area you will be in makes a difference if you will like your gears.
Are you going to be seeing:
Towing 5-9K lbs?
Mountain grades?

Both??

I went a little too far with my F700. however i have a 6sp manual trans so i only use 6th above 55MPH on highways.
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Old 10-21-2018, 06:22 PM   #10
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Quote:
Originally Posted by Mekanic View Post
Gearing up to a higher ratio(lower number ) and getting it exactly right is not easy the first time.

Getting the RPM you want is So very easy. pick the gear, speed, and tire height and you have it.



I went a little too far with my F700. however i have a 6sp manual trans so i only use 6th above 55MPH on highways.
I have the same trouble.

I have 3.73's in my truck with a 5.9, 6speed and 22.5's.

Backing up trailers is a pain. Reverse is "too fast".

Most of my highway driving is in 60mph limits. That makes for a lot of 5th gear driving.

I also get my best fuel mileage in 5th gear.

4.11-4.66 gears would be a notable improvement.

Taller is not always better...
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