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I have a 00, F550 superduty,2WD, 7.3, 6 speed trans. 4.88 gears. I use this truck for long distance towing. At 70 MPH. I am turning close to 3,000 RPMs. I am wanting to gear up to around a 4.11. Does anyone know if this is a good choice? I am wanting to gain fuel milage but dont want to lose alot of pulling power. Truck has straight exhaust and AFE stage 2 cold air intake. The rear end tag is in so bad of shape I cannot tell the size of rear only the ratio. Does anyone know if a 2 speed is available in this size rear?
 

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Well it is an F550, so its really not meant to be going 70 all day getting optimal MPG. 4.11's or 4.30's are a good drop down which would bring the RPMs back down a bit.

How big are your tires? If they are stock 31.4" tires, from my calculations you should be right at 80mph at 3000rpms. Gearing to a 4.11 would put it at 96mph at 3000rpms.

Not sure on the 2 speed rear, but i believe it is a dana 135 rear. Here is some info i just copy/pasted from another site:


There are 17 different ratios available for the Dana S135-S axle, including both the 3.91 and the 4.10 ratios that texas550psd asked about.

"Here are the Dana/Spicer part numbers for the matched ring and pinion sets on the ratios requested:

3.91 Ratio Ring & Pinion Set: 360KG105-X

4.10 Ratio Ring & Pinion Set: 360KG106-X


An important HOWEVER:

Since both the 3.91 and the 4.10 ratios fall outside of the 4.88 - 5.38 range of ratios the stock differential case uses in both diesel and gas configurations, a different differential case will also be required to use the taller gears. (By "case" I do not mean the external housing, but the internal differential case, ring gear backing plate, and bolt group.)

The Dana/Spicer part number for the differential case group appropriate for the range of axle ratios from 3.91 to 4.44 is:

360KQ102-X

Information Sources:

Spicer Heavy Axle and Brake Division
Fort Wayne, Indianna"
 

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I have a Dana S-135 rear end with a 4.88 ratio on my 99 550. I do mostly long drives with a light load that a 350 could handle. Can a gear set from a S-110 rear end work in the S-135? The 04 550's came with a 3.73 ratio option and both the 99 S-135 rear and the 04 S-110 gear have a 12 bolt mounting pattern
 
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