I have a 99 QC 4x4 5.9L, I just put some 35" nitto terra graplers on it. I was going to put some new gears in it, the sticker in the glove box says I have corporate 3:55's and I was goin to swap to a 4:10 however I am starting to think that the previous owner had already changed the gears out. Currently with the 35's on the hwy at 60mph I run as 1550rpm with the cruise control set. What ratio do you guys think I have.

 

W/ 4.56's, I'm at 2100ish at 60... I'd guess you're running 3.55's.

 

You need 4.56 minimum with 35" tires. I'd even strongly consider 4.88s.

 

Aim is right, alot of people say I over geared with my 4.88's but I absolutely love them. I assume that rpm is with od on? If so, leave it off. I never used od when I put my 35's on.