14F150inFL

My 2014 F150 has the 3.5L Ecoboost engine with a 3.15 axle ratio. As I read this chart, my RV/trailer GVWR is 8500 pounds. Why, simply by changing the axle ratio to 3.55, does the trailer GVWR increase by 1300 pounds?

TerryD64

It's all about leverage.
Taller gears mean longer lever and more PULL.

That being said, as has been discussed many times the real limiter on our F-150s is payload, followed by either by axle ratings OR tongue/hitch weight.

Unless you weigh 150 pounds and put nothing in the truck (except fuel) from when you bought the truck. You'll most likely run out of payload long before you ever hit the published max tow values.

smokeywren

Your question is confusing because you are calling the max weight of a trailer you can tow without exceeding the GCWR of the tow vehicle the GVWR of the trailer. But the max trailer weights in that chart are not the GVWRs of the trailers. Let me restate your question so it makes sense to me:

"As I read this chart, my tow rating is 8,500 pounds. Why, simply by changing the axle ratio to 3.55, does the tow rating increase by 1300 pounds?"

Tow rating is the max weight of any trailer you can tow without exceeding the GCWR of the tow vehicle. Tow rating is GCWR minus the weight of the tow vehicle. So if you increase GCWR without changing he weight of the tow vehicle, then you increase tow rating. GCWR changes with axle ratio, therefore tow rating changes with axle ratio. GCWR depends on the power and torque of your drivetrain, and shorter legs in the axle ratio means more power and torque available from the drivetrain. So if GCWR increases because of shorter legs in the axle ratio, then you can expect the max weight of the trailer to increase also.

Storeman

chimmike

They don't sell a truck with a gear ratio that keeps your vehicle outside the powerband. Not only does it not make sense to do that, it's bad for efficiency.
And with the 10sp auto, it is the great equalizer, and rear ratios don't mean nearly as much as they used to.

smokeywren

The key to understanding what the chart you posted means is to first understand GCWR (gross combined weight rating). GCWR is the maximum combined weight of tow vehicle and trailer you can drive without burning up anything in the drivetrain, and without being the slowpoke holding up traffic over hills and mountain passes. Notice that GCWR varies based on engine and axle ratio.

The tow rating = GCWR minus the weight of the unloaded tow vehicle with nothing in it but a skinny driver and only those factory options on the tow vehicle that are required to achieve that GCWR. Tow rating is overstated because it ignores GVWR and payload capacity of the tow vehicle. But tow rating is not the same as GVWR of the trailer. The GVWR of the trailer must be high enough to exceed the tow rating, but it can also be a lot higher than the tow rating. For example, your tow rating is 8,500 pounds, but you could tow a trailer with GVWR of 10,000 pounds without a problem as long as you didn't load the trailer to gross more than 8,500 pounds. Notice the chart you posted doesn't mention GVWR of the trailer, but instead has a column heading "maximum loaded trailer weight rating". Old heads around towing forums call the maximum loaded trailer weight rating the " tow rating" of the tow vehicle.

mass-hole

Larger numerical gears put less stress on the components forward of the axle in general. If you are in 8th gear at 65mph with 3.15's, and your trucks motor needs to produce 350 ft-lbs to drag that trailer, then a truck with 3.55's in 8th only needs to produce 310 ft-lbs due to the gearing advantage(3.15/3.55 = .89 * 350ft-lbs = 310 ft-lbs). This means the engine has to work harder and the transmission has to deal with more stress on the input and output sides with 3.15's.

With the 10 speeds transmissions it doesnt matter so much because of the abundance of transmission gears, but it still means that the transmission has to pick up the slack for the taller axle gears. The gear multiplication is now happening in the transmission which means everything after the two gears in the transmission is dealing with more torque(back half of trans, transfer case, driveshaft etc).

My guess is it has to due with the increased stress experienced by the transmission, transfercase, engine and other components forward of the axles. While they can do it with 3.15's, the long term durability may be affected. also, the transmission fluid and engine coolant temperatures may be affected as well.

BlackBoost

Imagine driving a bicycle, a 21-speed.
You have a bike trailer attached to it that has 300 lbs in it.
It would be easiest to pull it in the easiest gear right? And you could pull it for hours.
If you put the bike into the hardest gear, how long do you think you'd be able to pull that trailer before being exhausted? Not long, but you'd be able to pull less weight for a longer time.

That's why truck have gear ratios. Keeps the engine from being strained. Easier gear mean the engine doesn't have to work as hard, won't overheat as easy, and therefore can pull more weight.

Steve83

It has nothing to do with the engine getting tired or hot. It has to do with the available torque from the engine, the transmission's torque capacity, the weight distribution on the chassis, the tires, the suspension... So the chart lists those things in terms the customer can understand: engine (which implies transmission), cab, wheelbase, 2/4WD, & RAR.

BlackBoost

But it does. It's the engine's heat management. Run it too hard and it can't keep cool. They can both have the same engine, so the engine's torque is the same, it's the torque after the diff gearing that makes the difference. And one gear makes it harder to pull the same weight compared to the other. Weight distribution of the chassis, tires and suspension are all the same so I don't know where you're getting that from. They're both SuperCrew 145 WB 2WD...

The gearing is the only difference between the 2 circled lines, everything else is the same. The only thing different on that chart is the one at the bottom with the 3.73 gearing, but that one would have the thicker frame. Each config of cab and bed has 2 frame thicknesses available.