Posted by DRussell on October 20, 2021 at 10:26:52 from (137.200.32.54):
I have a 2000 F550 with 7.3 diesel, automatic (4R100) transmission and 4.88 (Dana/Spicer 135) ratio rear-end. I did not like how the 4.88 rear-end made the engine rev so fast at highway speed, (about 2600 rpm at 65 mph) so I got a rear axle out of a 2011 F450 with a 4.10 ratio (Dana/Spicer 110).
Initially, my plan was to swap the center sections between them as the 2011 rear had 10 lugs and my truck has 8 lugs and I didn't want to have to buy new wheels, but soon found out that the center sections will not interchange.
So, after doing some research, decided to switch the hubs on the axles and then swap out the entire rear axle under the truck. Changing the hubs went well. The bearings and brake components are the same on both except that on the 2011 the brakes are behind the axle and on the 2000 they are in front of the axle. This required minor re-routing of the brake lines and emergency brake cable, but as noted came off with no problems. I also had to lengthen the driveshaft about 2 inches and had to shim under the driveshaft carrier bearing in order to get the u-joint angles correct to not have driveline vibration as well. Engine speed at 65 now is about 1900 rpm.
Prior to the change I had pulled a trailer loaded with an Allis D17 tractor, the truck pulled it ok with the 4.88 rears, but would lug down too far on a hill before a transmission gear shift, the EGTs were hotter (1000 to 1100 degrees). After the change to the 4.10 rears I used the same trailer with a Massey Ferguson 65 with a loader. The D17 and the MF 65 with loader should be similar load sizes. I didn't weigh the loads. Neither tractor had loaded tires.
The truck actually pulled the trailer better with the 4.10 rears than the 4.88 rears. This was a surprise to me. I figured it would struggle more after the rear end change. However, EGT temps were lower (never got over 900), it shifted more naturally, everything just seemed easier.
Why...I've thought of a couple reasons. First, the combination of the engine power band/horsepower/torque, coupled to the transmission gear ratios combined with the rear end ratio is a sliding scale. I'm thinking the 4.10 rear-end ratio moves the engine to the lower end of the power band at a given speed which means it more quickly changes gears in the transmission rather than being at the top end of the power band and lugging all the way through to the bottom before a transmission gear shift. This makes for less lugging, which ultimately results in more natural gear shifting and lower EGTs. That being said, this doesn't totally make sense either.
The other reason is that maybe the ECM/transmission control module (probably not saying that right) settings are actually for a 4.10 ratio rear than a 4.88, or at least better suited to the 4.10 ratio. The truck has a chip which is supposedly set to control the transmission shifting sequences based on the 4.88 rear, not the 4.10 rear.
I don't know. It was different than I expected, maybe my expectations aren't right. Maybe some of the knowledgeable people on here can explain it.
This post was edited by DRussell on 10/20/2021 at 11:28 am.
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