Hey guys, i need some information about doing a gear swap in my truck. I need to change out the clutch packs for the limited slip and the pinion seal, so I figure it would be a good time to change gears too.

I have the 20" Roush wheels and 275/55/r20 tires.And those Roush wheels are HEAVY. I'm keeping the Roush wheels since the truck is a Roush. I want to do the gears mostly for performance. I drive 70% city and back road and 30% highway. I get about 17-18 mpg on average.

A few questions....
1. How will this affect my mpg?
2. It is an fx4. It's my daily driver, and I'm busy nearly every day except Saturday. So could I just swap my rear gears, and just not use 4 wheel drive until I have time to swap the front gears?
3. What gear ratio will be best? I'm thinking 4.10.
4. How will this affect my rpms ? I run about 1700 @ 65mph. About 1850 @ 70 mph. And about 2000 @ 70 mph.

These are really all the questions I have for now.

Thanks in advance for anyone that tries to help!

 

Nobody?

 

You're not going to get any better mpg than what you've already got.

Swap them both at once, you don't want to wait and then break something

I've got 4.10s and 285/70R17 tires. I'm right at 2000rpm at 65mph. Your highway mpg will probably decrease a little since my tires are an inch taller than yours

 

Because you're an FX4, you likely have 3.73s and I don't think 4.10s would justify the huge cost of doing gears. But if you it yourself like you seem to be, maybe it will be. It'd be nice if they made 8.8 reverse cut 4.30s for you 4wd guys, cause I'd want those if I were you, especially because a majority of your driving is city/roads. Generally lower (higher #) gears help city mileage and hurt highway because you're revving higher, but you do most of your driving on slow roads and can really sling it around it you want to.

gbctrdrman just threw 4.56s in his 2wd HD with the same (about) size tires and he loves them. I thought that was insane on a stock-tired truck, but apparently not. I almost want to see you do 4.56s and just get 275/60/20s for tires next time cause THAT will be a performance gain!

You're a non-supercharged Roush, right?

AK, I think he just wanted to know how his mpg would change, whether good or bad.

 

Yes it's a non-supercharged Roush, unfortunately. I really don't want my mpg to change much, but if the performance is worth it then it's fine. I am doing the install myself, and don't want to have to borrow someone else's car. So that is why I ask if I could just do the rear gears until I had time to do the front gears.

So really my only concern at this point is can I swap the rear and just not use 4 wheel drive? Without hurting anything that is.

 

You could, but if the vacuum powered hub-actuators failed (as they have been known to) or accidentally got locked into 4WD, you'd be in for an expensive repair for sure when it all bound up at speed. I don't know your mechanical abilities, but rear end gears and the associated shimming/lash adjustments are not for the novice. You could be throwing away $400 very quickly if you don't get it right the first time.

 

Hi OP;

First FX4 does NOT automatically connote a 3.73 rear - that's BS.

Post the axle code from yer driver side door/jamb sticker. Then we can go from there.

Stock tires - I'd do 4:10's if a 3.55, or 4.56's if a 3.73 ( or just leave it alone and buy an SCT X4).

Mileage will suffer slightly depending upon yer selection, but the silly grin on yer face will offset that quickly. 4.56's will literally tranform yer truck. Go big or go home

I also do not recommend only doing the rear. And unless you have access to expert help with the proper tools, I'd have a pro do the install.

There IS a feller on here who did it shadetree-style though - this feller: https://www.f150online.com/forums/ar...p-rev-1-a.html

good luck!


MGD

 

I said likely, not automatically. A majority do. Read more carefully

 

It's not even 'likely'.

Choose more accurate wording

In my demographic the majority of trucks were sporting 20's - which automatically selects 3.55's.

MGD

 

I haven't said it since, lesson & truth learned the hard way.

Funny thing I noticed is more XLTs have the B6 axle more than anything else. Also seems more Supercabs with 17s have the B6 too, better payload on those trucks I assume. I was pissed when my truck had the 3.73 option but not the LS. Have to take care of that someday.

x2. Better safe than sorry. 4.56s will make it run great for stop and go, probably save some gas. But not on the highway.

OP If you end up doing the rear only, disable/remove your 4wd switch from the dash for best measure. Better safe than sorry. If access is easy, I often unplug window switches when I tint peoples windows. Best security measure.

 

Darren - that will not address the potential IWE vacuum failure from locking the front hubs ( which is their default position. )

MGD

 

As long as the transfer case doesn't engage that won't hurt anything if the IWE's lock.

To be totally safe I'd pull the front driveshaft.

 

I'd be interested in actual % of each. From personal experience of my own truck and those of family and friends, most 04-08 2x4s have 3.55 and most 4x4s has 3.73s, so I will not "bow down" to your statement like you'd like me to.

My money is still on him having 3.73s.

 

Axle code is b6, so 3.73 limited slip. Maybe i should just wait a little while until I have the 1950 F1 back on the road (getting new floors, a c4 trans, and a hopped up 302) before I swap them that way I can do both at the same time.

 

Ha