I think you will be happy with the 3.55 its a good overall gear ratio i had a truck just like yours with that combo ie:351-e4od-3.55 locking rear and would get 16 on the highway !if you are looking to swap rears at a junkyard find a 8.8 rear with3.55 and factory locker this will be the best of both worlds

 

That's pretty much what I figured andym . Thanks.

Thank's Linemo.

 

Did these trucks come with lockers or did they only come with limited slip. I'v never heard of them having lockers. And im thinking about swapping gears and if im gonna do it im going to go ahead and put a locker in and it would be nice to have one from factory, but if they only came ls then id put in a real locker.

 

Just to give you a reference, my F350 crewcab, 5.8L, manual trans (ZF), 3.55 rear gears, typically gets 15ish on the highway, and 12-13 local if I'm reasonably gentle.

The last time I towed (from NJ to Ohio and back) I had about 6000lbs out back and averaged 12-13 on the highway, again shifting appropriately for the terrain and not romping the throttle.

My crewcab has a little over 400K on it now, original engine other than headgaskets, so it's a tired engine for sure.

Generally speaking, you want your gearing to put the lower range of the engine's torque band at highway speed in overdrive. Then for towing you can shift later, or stay in 4th or non-overdrive until you get up to speed, then shift into OD.

If your engine RPM's are at the top of the torque band, while you'll have more towing power you'll be sucking gas significantly more. If your engine RPMs are too far down in the torque band, you'll also suck a lot of gas as the EFI systems tries to compensate for lack of power/gearing by dumping more fuel into the engine.

There's a "sweet-spot" you want to try and find.

An option, which I happen to really like but haven't installed on my crewcab but have on other trucks I've owned, is to use a post-tranny overdrive/underdrive unit like gear venders. You get the unit that has normal/underdrive and use 3.08 or 3.55 gears, and unloaded you leave it in "normal" position for the best mileage while unloaded. For towing, you switch that unit into underdrive, which will hurt your mileage but basically simulates "on the fly" you swapping out the 3.xx gears for 4.11 gears. Pushbutton control, meaning you don't have to get out of the cab and you can switch the gear vendors unit at full speed under full load no problem whatsoever - it's designed to do this.