I need to change the left rear axle seal because it is leaking axle fluid and contaminated the left rear disc brakes. But how can I tell what size ring gear I have. My axle code is a 26 which tells me the ratio is 3.73 non ls. I also checked the rear axle tag and it says the following:

3 S-116 L
373 10 2 1D18

I have a 2001 F150 4x4 5.4 liter Supercab with the 7700 payload package. I think the size is 9.75 but then I read somewhere may be a 10.25, can't tell from the axle tag. Basically I need to know so I purchase the correct size seals before I take everything apart and realize I don't have the right size. I purchased Motorcraft part BRS118 from the Ford Dealership but they didn't seem very confident that it is the right one. I also did a part lookup by my VIN number on the Motorcraft website and it shows BRS118 as an inner wheel seal. I was unaware there is an inner seal, I thought there was only an outer seal. If anyone can help me identify the ring gear size or even the right part# I would be eternally greatful.

 

The best way to get the right parts is to give your dealer's partsman the tag number which in your case is S-116L.
The 10 and 2 means it's a 10.25" ring gear though.