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One thing you have to consider is which Allison 2500 do you have. There are 5 with that model number and they are engineered for different type vehicles. There is the 2500 General, 2500 Refuse (garbage trucks), 2500 Non-North America School Bus, 2500 MH (motorhome), and 2500 SP (specialty vehicles). They have different torque and horsepower ratings. There are also 4 different torque converters with different stall speeds. They are models TC-210, TC-211, TC-221, and TC-222. Below is a chart showing the part numbers. There have been 5 modifications on these and gas and diesel models are not the same. I believe the 210 and 211 are the gas models and 221 and 222 are for diesels. Transmissions also come in 5 or 6 speeds. Here is a general spec sheet. https://www.copelandintl.com/shop/on-highway/1k-2k/allison-2500-series/
 

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That Dana S135 was the strongest rear ever in a non medium duty truck. The Ford F550 used it for several years but they eventually changed to the S110. The S135 was just overkill in those trucks. What gear ratio does yours have?
 

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Discussion Starter · #46 ·
That Dana S135 was the strongest rear ever in a non medium duty truck. The Ford F550 used it for several years but they eventually changed to the S110. The S135 was just overkill in those trucks. What gear ratio does yours have?
Mine is out of a Chevy Kodiak 4500 with 4.78 ratio... It will require me to find a new 3rd member at some point so I can match my D60 front. My plan is to drive it for a while and then either go up to 4.88 or in the more likely case, down to 4.10 with the factory Posi traction unit.
 

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If I remember correctly, the S135 and S150 share common ground on the gears so there are all kinds of ratios available. To got to a higher ratio you'll have to change carriers. I believe the break points are 3.38-3.73, 3.91-4.44, and 4.78-5.38. 4.10 would be in the second carrier range and I believe the Ford may have had that. For years no one knew the S150 was the same.
 
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