While I have my front axle apart, I'm thinking I should to a complete rebuild, change gearing and add an ARB Air locker.

I was watching a video on gearing today. Using their formula, new tire size (37") times current gears (3.55) = 13.35 divided by old tire size (32") = 4.10.

Does this sound right? I am going from stock tires 235's to 37's. I have a few modest mods planned to get me to 400 HP and 600 foot pounds of torque.

 

It all just depends on what you plan on using it for. If it were me with such a tall tire I would deff think about doing at least the regear

 

I want to maintain what I have. It has reasonable fuel mileage, power and tows well.

 

Without knowing the actual 37 that you're going with your final effective gear drive would be around 2.84

 

If you aren't towing real heavy the 4.10 will get you then but more gear may be better really.
I know my 3.55s and 285 don't get 10k + moving from a stop great without torturing the converter..

 

For final drive ratio comparisons on your tranny if you actually math out a 3.55 or a use a 3.73 base ratio you come to a 4.36s.

The 2000 and up axles actually came with a 4.30 option. If I were you running 37s and towing i would not go with anything longer than a 4.30 ratio. A lot of trucks we have built have said this is nearly perfect. Some have gone as high as 4.56s. I can tell you personally that if I were to tow with 37s and 4.10s it would not be the best experience.... 37s on 3.55s... yeah forget it, unless you are hauling around a light camper trailer without the need for overdrive...

Also if you have a 400ho motor your more than likely up in the 900 ft/lbs of torque. You will fine out it will be very hard to keep a 7.3 cool yanking a trailer around at a 400hp setting with 37s under it. We just finished a build doing just that and it was not cheap!

 

Agreed I went from 3.55 to 4.10 in my f150 wasn't happy it made a difference but not enough so went to 4.56 because I couldn't get 4.30 for the front end

 

If you do the math with 325/80/16 on 3.55 then
Effective gear ratio would be 2.91
New gear needed comes to 4.33

 

You definitely want to go to 4.30 if you can get the gears. You don't have to look to hard to find guys with 4.10 gears and 37" tires who aren't too happy with the combo.

 

Well I'd get a set of 4.30s if I were you and call it a day...

 

My f150 is 4wd can't get 4.30 for the front last I checked it's got 4.56's in it for a couple years now no complaints with that gear setup

Edit: now I realize you were probably talking to the op lol