1. Driver's side doorjamb: This is the most common location for the sticker containing the vehicle's specifications, including the axle ratio.
2. On the glove box: Some models may have this information printed on a sticker inside the glove box.
If you can't find it in either of those spots, check the owner's manual. It might list the axle ratio there. Failing that, you could check the vehicle identification number (VIN) plate, though the axle ratio isn't always listed there. As a last resort, you can look at the differential itself, but this requires some mechanical knowledge and getting under the truck.
