If I'm on dirt and put the pedal down it just gets sideways or spins,still has some good power but I have the 5.4.

Honestly 4.56 will get you back to stock rpm range, actually a hair higher...

If you are looking to go off road and sink yourself in the mud, 4.88 will be better but on the street will be a bit on the peppy side... I would not go 5.13 with 37s and a 4.6 on the street, leave that for 40s+...

I ran 37s and 4.56 on my 5.4 a while back... it would spin the tires at half throttle just trying to get into traffic... and was a bit too peppy on the street, had to be easy on the throttle or I'd be replacing tires sooner than I'd like...

Yeah so to me it sounds like 4.88s are exactly what i want :D

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I'm running 37's with a 6" Zone. 5.4 (slightly built & a custom tune) still have the factory 3.55s in it but getting 4.56's soon & mounting a M112 charger on her. With yours being the 4.6 I would go with 4.88s. You may loose a lil off the top end but she'll pull much better. As far as clearance, I had to trim my front valance & fender slightly so it wouldn't rub.

New tire size x stock gear ÷ stock tire size = proper gear to get rpm range back in check...

37x3.55÷30=4.37

Closest available ratio = 4.56 which would bring you above stock rpm range...

After getting rpms in check and you want to go bigger, you can use the equation with the stock numbers again...

40x3.55÷30=4.73... available gear 4.88, again above stock range...

And so on, always use the stock numbers after the new tire size... that way it doesn't get progressively higher while changing...

Don't bother thinking about what you have now... you're probably 500 rpm low... remember where the power was on stock tires? 4.56 will get you back there and then some on 37s...

Like that formula. Nice post.

Hahaha, remember in school when you asked "when am I gonna use this in real life"

Wellllll.... Lol

I found myself using trigonometry building my truck a few times haha... math is a good thing, takes the guess work out for a definite answer... always works...

very sharp. I'm not a big step side guy but done very tastefully.

Lol nice formula. All the tire charts say 4.88s though I feel like 4.56 would be undergeared and from what everyone tells me 4.88s would be right on 37s. But math doesn't lie lol

Yes, 4.6 sunk in the mud, 4.88 would be perfect... if it were the 5.4 I'd say 4.56 in the mud...

On the street, 4.88/4.6 is gonna be close to too peppy but I don't think it will bother you... unless you do a lot of highway...

If you need help setting up your gears, I have done many on ford corporate axles and others... the right way... let me know, if you're unsure, definitely bring it to a professional, and I mean a shop that specializes in the field... I know some mechanics that think they know how to set up gears and they don't have a clue... they get em close enough but that's not good enough for me... perfection is key to longevity...