Assuming the truck is still running the stock gears you could look at the axle code on the door sticker & do a Google search for Ford axle codes to decode it. Another way is there is a tag on one of the rear diff. cover bolts that will have the info on it. The hardest way is to count driveshaft rev. / one tire rev ( both tires need to rotate at the same time ). The final way is to pull the cover & look for the tooth count stamped into the side of the ring gear or physically count the ring & pinion teeth.
essentally 4.10's will be quicker out of the hole and better for towing un loaded acceleration isnt really noticable imho over a 3.55 but they will give you lower rpm at high way speeds and you will get better mileage
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