The outcome was different than expected-long post

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.

I'm certainly not in the knowledgeable persons category but it sounds like a winning situation to me. Happy for you.

I am curious if your speedometer is now reading the correct speed. If the sensor is in the transmission tailshaft, I would think it would read speed for the old ratio, and thus you are running faster than you are reading. Mark.

I am no expert at all on this but it sounds like you hit the sweet spot of the engines power band. Congrats.

Speedometer reads off of the rear-end. The sensor comes out the top over the pinion.

It was off by a mile an hour or two previously which I attributed to slightly bigger tires. 245/70-19.5 instead of 225/70-19.5s.

Now, with the 4.10 rear-end it is dead on compared to the GPS on my phone.

One way to check your speedometer is time the travel between mile markers on the highway at 60 would take one minute. Mileage should also change each mile marker too. If either are off you are not set for the correct reading. Another words at 60 you would travel a mile in a minute and your odometer should click one mile reading in that mile. If the mileage reads different more or less the speedometer is still off and mileage will be off for fuel tax. Don't want more miles to pay more tax than needed and don't want to be to far off short or might question your mileage against the fuel tax. Don't know if the fuel tax is an issue in your operation or not. I've been dealing with this on a new speedometer head for the semi. I measured the travel of one revolution of the tire several times used the companies chart and wind up either to slow or to fast. Right now it reads about 3 mile per hour fast for the actual travel speed. this is according to what the engine speed has read for the last 20 years in the truck and the original speedometer read.

I measured it by using the GPS for the maps app on my phone. When you open that up down at the bottom of the map it indicates how fast you are going. The GPS is pretty accurate. You have a real time comparison then between what the GPS says vs what the speedometer on the vehicle says.

While your HP will continue to increase to roughly 3000 rpm your torque peaks around 2000-2200 rpm then begins dropping off.

The changes you have made better match the rpm range where peak torque is being produced for the (65mph) speed you choose to drive.

With the 4.88 gears the truck would have been optimal around 50-55 mph.

Determined, I'm no expert on torque and hp curves, but I was wondering if something like this was
the actual explanation.

Changing from the 4.88 to the 4.10 rears made a world of difference in the truck. It's way more
pleasant to drive now. Even my wife commented from the passenger seat how much of a difference it
made.