I have a 2004 F350 dually 4x4 with 4.10 limited slip rear & transaxle. What are the potential problems if I change both to 3.73 gears? Would this affect the shift points of the tranny (5spd auto)? speedometer/odometer readings? torque/performance of pulling 4horse/living qtrs. gooseneck trailer (10,300 lbs. empty, 16,000 w/4horses)? any significant change in mpg when not towing? any & all info greatly appreciated!

 

The ratio is about 9% different, so you will lose that much torque, but you may see a little increase in mileage. The transmission shift points are partly determined by speed and load, so if the engine computer detects greater load due to the decreased gearing, it may shift later. And yes, your speedometer will be off, unless you recalibrate it.

And I don't think you have a transaxle; that's where the transmission is integrated with the axle. Your transmission goes through a transfer case first to split the drive power to the front and rear axles.

 

Now for a totally different answer.

The computer is smart enough that it will adjust the shift points so that it will still shift at the right speed. The speedometer will be unaffected. The speed sensor is reading axle shaft speed, so that does not change when you change gears.

xlt4wd90 is correct that you will lose 9% tractive effort. Is that enough that you will notice the difference? Maybe. Some drivers will, some will not. I don't know where you fall in that list.

If you are frequently finding yourself with the go pedal all the way on the floor, then the loss of 9% will be more noticeable. If you rarely are on the floor then you probably won't notice much difference at all.

Any change in fuel economy will be small. You probably won't ever drive the truck enough miles to save enough on fuel to pay for this swap.

And no, you do not have a transaxle. You have a transmission, then a transfer case, then front and rear axles.