So I am trying to figure out what gear ratio I have in the truck in my sig. I have the build sheet from the dealer and it says 3.55's and a dana 80 hybrid. I am confused though because at 65 mph. I am in the1850 - 1875 rpm range. Shouldn't it be lower if I had 3.55's? I don't know much about differentials or gearing so I am just trying to learn something new :-)

 

Sounds like you do have 3.54s in that truck with that MPH and RPM you list and the 285/75R16 tires.
Your manual truck would have come with a Dana 80 rear axle.

 

Cool thanks for the confirmation!

 

Yep, definitely 3.55's. Similar RPM's to mine. My observation, great for mileage, not so much for towing heavy.

 

There's no issue running 285/75s with that gear ratio is there?

 

I don't think so. It will drop your cruising RPM's a little (65 RPM less at 70 MPH), and you'll take a very small hit in towing ability.

Most days I like having 3.55's. When towing the camper, I really wish I had 3.73's or better. I am running 265/75 tires.

 

Ok I was jw because I bought my truck with 285's but switched to 265's because that's what the build sheet says it came with. But then I had a warranty issue and decided to go back to the 285's because there is plenty of room and I like the way they look and I was thinking they might help my fuel mileage a little....but I didn't think about the gearing and if it would hurt anything....

 

Nope, won't hurt the gears. Only thing you might hurt is the limited slip in the rear axle (if you have one, check the sticker in the glove box to find out) if you carry the 265 spare. Putting a 285 and a 265 on an axle with a limited slip will eat up the clutches in the differential.

 

ok thanks. I do have a limited slip according to the build sheet. I won't mix tires on the rear axle.