Ok, I'm planning on modding my new 08 fx4 supercrew with a few things and I've heard a few people recommend changings my gears? What exactly does this do, and does it increase performace, or..?

My setup includes:
Procomp 6" lift
35" tires
20" rims
Maganflow SIDO rear cat-back
AF1 CAI (maybe 3.5")
(possible X3 TP tune)

So based on those specs, what would you recommend for the gears? If so, why?

 

You need 4.56's.

 

Go 4.88's. I've got 4.56 with stock height tires. What you do when you put a taller tire on is increase the load on your engine by effectively reducing the over all ratio for the vehicle. To bring back the right ratio you change the gears to a numerically higher ratio. It has changed mine to where it's allowing the engine to rev up faster and get to a more powerful part of the power band. With a 35% faster ratio the torque of the engine is more than the old tires can handle and break loose from a dead stop when giving it a lot of power.

 

Wow 4.56's and stock tires, now thats crazy IMO. Most people run 4.56's with the 35's, so thats what I suggested. How is the mileage on the highway and what rpm do you turn? My 3.55s turn 2500@ 95mph.

 

It's a 35% increase in rpm and the overdrive is a 30% reduction. I went up an additional .7 mpg also. I run a 93 octane tune with a lot of spark during cruise. It gets second looks when I accelerate or brake. A truck that heavy ain't supposed to do that.

 

What does the O/d reduction have to do with it? 3rd is a 1.00:1 and 4th is .70:1. I do alot of highway driving and there is NO WAY that you got better highway mileage running 4.56's over the stockers(3.55/3.73). And for the 93 and "alot of spark", are you saying you run increased timing, which once again has nothing to do with the gear swap. I agree that the truck will accelerate like a bat outta hell, but the braking as well. Your post just confuses me, and thats very hard to do. So what is your RPM at 70 mph in O/D, and do you know where the truck shifts out of 2nd at WOT(MPH)?

 

Since I've been tuning the truck likes to not use the EOD with the 3.55's. To run reasonable rpm's now above 35 I use EOD and it's about like running 3.55 in high gear. I am at 2425rpm's at 50mph now in high gear. The higher spark advance is up around 40 at cruise and 20 on acceleration. Motor seems to love it. Tons of throttle response and pickup. Brakes have been upgraded to steel Baer rotors and EBC pads.

 

Wow thats alot of rpm @ 50mph, def. couldn't have that setup. Your timing is odd as well, mine runs 30 at WOT about 25 at cruise, but you also have PI heads, and better heads require less timing advance. I got ya with the brakes now, makes more sense.

 

Now you see why I'm not a technical writer.