Kansas Transit

All of the above comments have summed it up pretty well, the only thing I will add is that in this day and age, it just really isn't near the big deal it used to be.


The more gears they put in the transmission the less critical it is to get the rear axle "perfect".


In days gone by it wasn't uncommon to have gear ratio's available from the factory that ranged from 2.50's to 4.50's BUT you only had 3 gears in the tranny.


Now you are down to 3.15 to 3.73 in most cases, tranny ratio's, fuel inj. computer calibration, 6/8 speed tranny's and electronics have all contributed to the rear gear being less of a factor.


As others have stated if you are going to larger tires, or a max tow rating or ALL of your driving is at 70mph anything you get will work.


Truth be told, most people couldn't tell the difference between 3.15 and 3.31 the gear multiplication in the tranny, still gives you more than adequate starting torque.


For instance I just bought a 2014 5.0 off the lot as time to order had expired, I was sure that I needed a 3.55 but the closest truck I could get that had everything I wanted only had a 3.31, and to be honest, it's perfect.


Hope that helps.


Stan

justjimmy

I couldn't agree more but I don't believe the 2015 EcoBoost is available with a 3.73. Too bad because, everything you said is spot on.

papa tiger

Believe the 12500 lb Max Tow rating is only available in King Ranch. Guy doing tow comparisons says that.

Kansas Transit

That kills me PT, what if you wanted a "real" work truck? You know, like it was going to get dirty, muddy, wet and had crappy clothes on when you finished the job and then jump in the truck.


You have to spend 60 G's to get the Max Tow, It just seems really dumb to me.

dkj313

5.0. RCSB w 3.73. Good combination
But would you do the fx4 package plus the 3.73
Or the tow package and 3.73

Only tow light trailer. Want to put on larger tires.

GearheadGeek

The number I've seen in multiple articles is 12,200 lb, and tied to the Max Tow package which is available on most trim levels. You won't have to buy a King Ranch to tow big, which would likely not reach those towing numbers anyway since payload limits play into it too and the posh trims generally end up with less usable payload.

papa tiger

I noticed the 3.73 axle option on the truck I built. Think max tow gets the 51 tube radiator core so I would just ask the salesman. I built the 2.7L with max load an tow haul. 3.73 locker with tow/haul an crew cab 4X2.

Ron AKA

I think you have been given enough information on what the ratio does. I would suggest that the old rules of what ratio is the best for this and that all changes if you select the EcoBoost engine. As others have said, you would tend to pick a lower ratio if you wanted maximum fuel economy and don't tow often. You would pick a higher ratio if you tow heavy loads, or really want a red light street racer instead of a truck.

However with the low end torque of the EcoBoost you can kind of have it both ways. A lower ratio still has lots of torque for towing, and can give you better fuel economy when not towing. My planned configuration which is to be mainly used for towing a 5th wheel trailer is a 3.5 EcoBoost with a 3.31 rear end. On the other hand if I was to go with a 5.0 L V8, especially the higher tuned 2015 version, I would go with a 3.55 or even a 3.73. A 3.73 is what I currently run for towing. The Ford towing charts give you a good idea what each ratio is capable of considering the engine type and rear end ratio.

Have a look at the torque vs rpm curves at the link below. Notice how much difference there is between the 3.5 Ecoboost and the 5.0 L V8 in the 2000 to 2500 range. A low ratio rear end would put you into the lower range on the highway. The EcoBoost is going to love it, while the 5.0 is going to struggle. That is why the peaky 5.0 engine needs the higher ratio rear end to get you further up the rpm torque curve. But keep in mind that even with a high ratio rear end, the 5.0 is not going to come close to the 3.5 EcoBoost with a lower ratio. That is what the turbo gets you.

http://smg.photobucket.com/user/jonb...orque.png.html

And then consider of course that big wheels/tires screws everything up and effectively lowers your rear end ratio. To figure out by how much just multiply your truck rear end ratio by the circumference of the stock tires divided by the replacement tire circumference. That will give you the effective ratio with the replacement tires on it.

papa tiger

There is a reason Ford puts 3.73 axle in Max tow and the 51 tube radiator instead of the stock 41 tube one. U can probably get the larger radiator with every 3.73 axle ratio an tow haul.

L.T.

Could someone please explain difference between non-limited slip and electronic locking? Looking at KR with the 3.5 Eco and would prefer the 3.31 given my long daily commute vs. potential need for max towing.