3:31 Gearing

[cliffy49]

Looking into the possibility of a new to me TV. I just looked at a 2012 F250 with a 6.7 and 3:31 gears.

What I would like to know is if anyone here tows with this set-up and how it works. I realize that 3:31s are not normally the ideal choice for towing but since this is a diesel, it may be different. I realize that 4:10s or 3:73s would be a better choice. Buying used you do not always have a choice of the perfect unit.

The truck appears to have towed since it does have what I guess are part of the mount for a fifth wheel hitch in the bed. Also it appears to have the tow package just not the best set of gears.

Not trying to start a feud as to what is best, just trying to get comments and advice on this set-up. Thanks in advance for all of the help.

[4x4ord]

Thanks for the translation. I agree that 3.31 will work just fine and in fact will work even better than a deeper ratio when towing lighter trailers.......That is why it is offered.

[Grit_dog]

^Haha, yeah maybe not the best saying to use when talking transmissions and gears! Lol
Either way, 4xford, the confusion is your interpretation of "best" compared to others'.
I understand what you're saying and you're talking "best" balance maybe between ultimate capability and unloaded economy.
But that does not negate that deeper gears will pull harder all other things considered. You can cherry pick when a taller gear will be in the powerband for a given speed and pull harder and compare that to a lower gear, but that does not consider the fact that as torque multiplication increases so does pulling power. Period.

[rjstractor]

4x4ord wrote:

Grit dog wrote:
This has to be some of the dumbest schitt I've seen on the interwebs.
Picking a speed where a certain tire size, load, gearing and power theoretically work better than the better option?
I made a post about people who would say that toothpaste is bad to use with toothbrushes a couple days ago.....
I could say the same here.
Back to reality, OP, 3.31s...dog will hunt. Don't worry.
But deeper gears trump taller gears, all else equal....fact.

You are not understanding the concept of choosing a gear ratio. If deeper ratio gears trump the taller ratio gears the manufacturer would only offer the deeper ratio. There is a range of operating conditions where a specific gear ratio shines. If you don't understand the concept it's very simple ..... Choose the gear ratio the engineers recommend for the weight of trailer you plan on towing. The recommendations are based on math.

You say 3.31 dog will hunt .... If hunting is not allowed then choosing a 4.10 ratio and a dually for an 11000 lb trailer would be the thing to do..... You could pull any hill on the interstate in 6th gear. What are you going to do when the 10 speeds come out to prevent hunting?

:S

Time for a little American to Canadian translation....When Grit says "dog will hunt", that's an American colloquialism that means "it will work just fine". Similar to how Canadians refer to a situation that has gotten ridiculous and out of control as a "gong show", just like this thread.

[Lynnmor]

womps wrote:
I have the 3:31 ratio and my truck doesn’t hunt pulling my 14,000 plus pound 5th wheel. My truck has 925 lbs of torque. Uh, I wonder if that has something to do with it?

All that torque will wear the rear tires down faster. After they wear 1/32" the gearing will be way off and you then need to replace the tires or truck.

[womps]

I have the 3:31 ratio and my truck doesn’t hunt pulling my 14,000 plus pound 5th wheel. My truck has 925 lbs of torque. Uh, I wonder if that has something to do with it?

[4x4ord]

Grit dog wrote:
This has to be some of the dumbest schitt I've seen on the interwebs.
Picking a speed where a certain tire size, load, gearing and power theoretically work better than the better option?
I made a post about people who would say that toothpaste is bad to use with toothbrushes a couple days ago.....
I could say the same here.
Back to reality, OP, 3.31s...dog will hunt. Don't worry.
But deeper gears trump taller gears, all else equal....fact.

You are not understanding the concept of choosing a gear ratio. If deeper ratio gears trump the taller ratio gears the manufacturer would only offer the deeper ratio. There is a range of operating conditions where a specific gear ratio shines. If you don't understand the concept it's very simple ..... Choose the gear ratio the engineers recommend for the weight of trailer you plan on towing. The recommendations are based on math.

You say 3.31 dog will hunt .... If hunting is not allowed then choosing a 4.10 ratio and a dually for an 11000 lb trailer would be the thing to do..... You could pull any hill on the interstate in 6th gear. What are you going to do when the 10 speeds come out to prevent hunting?

[dodge_guy]

4x4ord wrote:

dodge guy wrote:

4x4ord wrote:

Lynnmor wrote:
Ford uses the 3.31 with 18” wheels only.

The Idea that 3.31 will out pull the 3.55 when both have the same tire size is absolutely incorrect.

If you read and understand my explanation above you will see that a 3.31 with 18" wheels can certainly out pull 3.55 gears and 20" wheels. The scenario above would exist with about a 15000 lb trailer going up a 4% grade. The same sort of scenario would take place in third gear with a 15000 lb trailer on about a 6-7% grade.

So on a 7% grade the trucks will drop into 3rd gear with 15000 lb trailers in tow. The 3.31 geared truck would rev to 2900 rpm and be travelling 55 mph..... the 3.55 geared truck would be travelling 52 mph at 2900 rpm.

You never mentioned 2 different wheel sizes until now. Of course they would tow the same, the final drive ends up almost identical. Now take the same size wheel and 2 different gear ratios and the lower geared one will out perform the higher geared one.

With the same size wheel and two different axle ratio the same scenarios can occur. The idea is this: A truck makes optimum use of its engine's HP when the gearing perfectly matches the load such that the engine is 100% loaded at the rpm where peak HP is obtained.

So consider two identical trucks other than rear gears.... both are 18" tires.

Truck 1 has 3.31 gears. At 2900 rpm it travels:
75 mph in 4th gear
57 mph in 3rd gear

The truck with 3.55 gears will travel:
70 mph in 4th gear
53 mph in 3rd gear

So if the hill incline/weight of trailer require full power in the range of about 55 to 63 mph or about 74 to 77 mph the 3.31 gear ratio has the advantage.

If the speed/weight combination are such that full power is required at 50 to 54 mph or 65 to 73 mph the 3.55 ratio will have the advantage.

Nope! the final drive effectively changes the ratio of each gear in the transmission. so the truck will not work as hard when starting off or moving down the highway in any given gear.

[Grit_dog]

This has to be some of the dumbest schitt I've seen on the interwebs.
Picking a speed where a certain tire size, load, gearing and power theoretically work better than the better option?
I made a post about people who would say that toothpaste is bad to use with toothbrushes a couple days ago.....
I could say the same here.
Back to reality, OP, 3.31s...dog will hunt. Don't worry.
But deeper gears trump taller gears, all else equal....fact.

[4x4ord]

dodge guy wrote:

4x4ord wrote:

Lynnmor wrote:
Ford uses the 3.31 with 18” wheels only.

The Idea that 3.31 will out pull the 3.55 when both have the same tire size is absolutely incorrect.

If you read and understand my explanation above you will see that a 3.31 with 18" wheels can certainly out pull 3.55 gears and 20" wheels. The scenario above would exist with about a 15000 lb trailer going up a 4% grade. The same sort of scenario would take place in third gear with a 15000 lb trailer on about a 6-7% grade.

So on a 7% grade the trucks will drop into 3rd gear with 15000 lb trailers in tow. The 3.31 geared truck would rev to 2900 rpm and be travelling 55 mph..... the 3.55 geared truck would be travelling 52 mph at 2900 rpm.

You never mentioned 2 different wheel sizes until now. Of course they would tow the same, the final drive ends up almost identical. Now take the same size wheel and 2 different gear ratios and the lower geared one will out perform the higher geared one.

With the same size wheel and two different axle ratio the same scenarios can occur. The idea is this: A truck makes optimum use of its engine's HP when the gearing perfectly matches the load such that the engine is 100% loaded at the rpm where peak HP is obtained.

So consider two identical trucks other than rear gears.... both are 18" tires.

Truck 1 has 3.31 gears. At 2900 rpm it travels:
75 mph in 4th gear
57 mph in 3rd gear

The truck with 3.55 gears will travel:
70 mph in 4th gear
53 mph in 3rd gear

So if the hill incline/weight of trailer require full power in the range of about 55 to 63 mph or about 74 to 77 mph the 3.31 gear ratio has the advantage.

If the speed/weight combination are such that full power is required at 50 to 54 mph or 65 to 73 mph the 3.55 ratio will have the advantage.

[dodge_guy]

4x4ord wrote:

Lynnmor wrote:
Ford uses the 3.31 with 18” wheels only.

The Idea that 3.31 will out pull the 3.55 when both have the same tire size is absolutely incorrect.

If you read and understand my explanation above you will see that a 3.31 with 18" wheels can certainly out pull 3.55 gears and 20" wheels. The scenario above would exist with about a 15000 lb trailer going up a 4% grade. The same sort of scenario would take place in third gear with a 15000 lb trailer on about a 6-7% grade.

So on a 7% grade the trucks will drop into 3rd gear with 15000 lb trailers in tow. The 3.31 geared truck would rev to 2900 rpm and be travelling 55 mph..... the 3.55 geared truck would be travelling 52 mph at 2900 rpm.

You never mentioned 2 different wheel sizes until now. Of course they would tow the same, the final drive ends up almost identical. Now take the same size wheel and 2 different gear ratios and the lower geared one will out perform the higher geared one.

[4x4ord]

Lynnmor wrote:
Ford uses the 3.31 with 18” wheels only.

The Idea that 3.31 will out pull the 3.55 when both have the same tire size is absolutely incorrect.

If you read and understand my explanation above you will see that a 3.31 with 18" wheels can certainly out pull 3.55 gears and 20" wheels. The scenario above would exist with about a 15000 lb trailer going up a 4% grade. The same sort of scenario would take place in third gear with a 15000 lb trailer on about a 6-7% grade.

So on a 7% grade the trucks will drop into 3rd gear with 15000 lb trailers in tow. The 3.31 geared truck would rev to 2900 rpm and be travelling 55 mph..... the 3.55 geared truck would be travelling 52 mph at 2900 rpm.

[Lynnmor]

Ford uses the 3.31 with 18” wheels only.

The Idea that 3.31 will out pull the 3.55 when both have the same tire size is absolutely incorrect.

[4x4ord]

Lynnmor wrote:

4x4ord wrote:



A lower geared truck does not tow better than a properly geared truck. For example if a 3.31 axle ratio can tow a 15000 lb trailer up a hill at 75 mph at wide open throttle It will be out doing a 3.55 equipped truck that is otherwise identical towing the same 15000 lb trailer up that same hill.

Now that does not make sense.

The truck in question has no real options. The two gear ratios available are very close and factoring in tire size there is virtually no difference. While some rail against the gear ratio available, there is absolutely no issue and folks need to understand that the 6.7 diesel will pull just fine as designed.

While I am pulling not much more than half the maximum tow rating, I top many of PA's finest four lane hills without dropping out of 6th gear.

Not dropping out of 6th gear is not the goal. Matching gear ratios to load to HP and expected incline is what the engineers do. The overdrive gears are intended to be used for economy .... cruising while high power is not required. Hit a 6% grade on the highway and if you have the right gears you should be in 4th gear.

The difference in engine rpm while traveling 60 mph between a 3.31 equipped truck and a 3.55 equipped truck with 20" wheels is a little over 4%. The 3.55 truck runs 2520 in 4th gear and 60 mph and the 3.31 equipped truck runs 2415.

So if the 3.31 truck is pulling at maximum power (2900 rpm) it will be traveling 72 mph in 4th gear. The 3.55 equipped truck will only be traveling 69 mph. If the 3.55 truck were to up shift to 5th its engine's rpm would drop to 2170 rpm and the truck would slow down.

So the lower speed gear ratio is not always going to out pull the higher speed ratio truck.

[dodge_guy]

A truck with lower gears towing a heavy load will get better towing mileage than a similar higher geared truck. The performance is also better.

You can’t change the laws of physics!

[womps]

Huntindog wrote:

womps wrote:

Lynnmor wrote:

cliffy49 wrote:
Well folks, now that I am totally confused:h:?. I am still pondering the Super Duty with the 3:31s but will continue to look for the "perfect" truck for my needs. Not sure exactly what that is but I guess I will know it when I find it.:B

I want to thank everyone for all of the suggestions and advice, it is greatly appreciated.

Whatever you decide, you need to get that 3.31 thing out of your mind, it is absolutely no issue. Zero, Zip, Nadda.

X2.

Balderdash.
The rules of physics haven’t changed.
A lowered geared truck will ALWAYS tow better than an identical taller geared truck while consuming more fuel. The reverse is also true.
One may be ok with the lower performance of a taller geared truck, and that can be ok. Just stop pretending that there is no difference.

Don’t care about the law of physics. I towed a 14,000 pound 5th wheel with both ratios and like I said couldn’t tell the difference. If ordering a new truck tomorrow I would definitely get the 3:31 ratio.