Hello
I just installed a 12" Rize lift with 37 inch Boggers on my 07' F150 FX4. The calculator that I used on 4wheelparts showed that a 4.40 be what I need. The next thing up is 4.56 and that may be what I want since I'm going up to a 38" tire in a couple of months. The problem is that I can't even figure out for 100% sure which differentials I have. I've been to every web site and all forums but this one and they all say look at the bolt pattern or the tag. Well I can't read the tags on my rear or front differential because of excessive paint. It's a 12 bolt rear end if that tells helps at all. I know for sure that I have 3.73's but no clue as to which differential I have. Can anyone help me out?

 

From all the research I've done:

4.56 for 35" tires
4.88 for 37" tires

9.75 rear end

 

Ok thanks but is the front diff. 9.75 also? Do you really think 4.88's would be better for 38's? What about on the highway? Will my rpm be really high.

 

Front is 8.8

Here is a chart to help choose your gears:

http://www.4lo.com/calc/geartable.htm

I'd go with the 4.88's for sure.