I did a search in here, no luck at all. I googled it and got so much nonsense (well, shucks Imma turning 1,200 RPMS with mah 44's), I just decided to ask real world folks.

Mostly with the 3:31 gears and the 3:55 setups.

If you don't mind.

Stock tires and rims, please. 18's and 20's

Thanks.

 

The reason I'm asking is, there seems to be two rear end gear sets for the F-250 Diesel; 3:31 and 3:55

We're all used to the numerically higher gear ratio being more gooder for towing, etc

But I wonder.

On a gasser, yeah. They don't hit their torque curve until much later. Some of them well into the 4,000 RPM range.

But with Diesels, the torque comes on much sooner. Likely more because of turbo.

And with the new transmissions that have two overdrives, and I believe the 6R140 will go into lock up in the top 4 gears.....

I wonder if there's any drawback at all in getting the 3:31 diff??

If you're doing 70 MPH and and towing at 1,800 RPM, aren't you in your maximum torque range?

And if you're empty and want to cruise at Warp 1..........??

I just don't know. I think it's a legitimate question for the prospective SD buyer.

Enquiring minds..........

 

I haven't noted it at 70, but at 55 it's 1300 rpm with 3.31 gears and slightly oversized tires - 285/70-17s. RPM in 5th is 1600.
This is on my current 2014 F-250.

 

This is from a trip I made the first week of March. '17 F-250 Lariat Ultimate, 6.7, 3.55, 20" wheels, FX4, + other assorted goodies to help make it heavy as hell. I was towing a 24' gooseneck that weighs #4000 lbs + with a 20' shipping container strapped to it weighing in at 4800#. To top it all off the height was 12'2", so it was like towing a big wall. The mileage didn't vary more than +/- .6 mpg's the whole trip. That was varied terrain, from north Florida through Ga, SC, NC, Tenn, Ky, Oh, and Indiana then back home the same route. Also, on the return trip I had a 4500# CNC machine in the container. Hope this helps.

 

That looks optimal to me. I'm impressed

I wonder if the 3:31's would lower the RPMs maybe a just little bit too much?

I take it you took I-77 up North? Then traveled around a bit??

Nice road anymore. I used to travel it through the Blue Ridge Mountains when the only thing between your vehicle and a 1,000 foot drop straight down was a rather flimsy looking guard rail.

Last time I came back from Michigan, I took I-65 then picked up I-75

That ain't gonna happen again. I-65 just simply sucks. And 75 ain't much better. But 77 is quite a bit out of the way.

Anyway, thanks for the information. It helps

 

I think you'll want to know the tire size as well, not just if it's 17",18" or 20". The tire diameter varies with the aspect ratio and that directly effects the overall final drive.

I was originally going to order an XL which comes with 245/70-17 = about 31.5". Pretty small for such low low gears. Knowing I wanted the 4.30 I was going to option up for the 275/70-18 = 33.2" which is about a 5% difference which will basically increase the rear end gear ratio (numerically lower). By the time I put on 35's I'll be around 3.90 so now those 4.30's are not all that low. my 2 cents....

 

Per the chart on this page, I'm not sure a few hundred RPMs is gonna make much difference for towing, as the peak torque is between 1600 - 2000 RPM, pretty much no matter what the model year of the 6.7L. This diesel has some snort.

6.7L Power Stroke Diesel Specs & Info

 

Personally, I wouldn't do the 3.31's. I think the 3.55 keeps it in a good spot at highway speeds. It doesn't hunt gears to keep pace up moderate inclines. Even on I-40 through between Ashville and Knoxville it only kicked down to 4th a few times. I took basically the same trip this past week without the trailer and got 18.1 over 1700+ miles, mostly interstate but running a pretty steady 80 mph (when traffic allowed). The route to Indiana was 95/26/40/75/74 then 31 north out of Indianapolis. I only went as far as Cincinnati last week.

 

Thanks. What part of North Florida are you leaving from? I try to avoid Cincinnati if I can. MY GPS took my over the Ohio River West of Cincitucky last time I went up. Wasn't too bad. I was heading to Michigan between Lansing and Grand Rapids.

I'm over on the West Coast, Fort Myers.

I HATE 65 and I'm not at all fond of 75. Wonder if it would do me any good to cross the State on I-10

Looking at your dash gauges at 6.8 MPG, and it looked like you were 'on the whistle' just a tad.

bookin'

 

The RPM changes based upon tire size and axle ratio. All 6.7L diesel transmissions have a 0.67:1 6th gear ratio.

So with some math, we can come to the following conclusions:

With 20" rims at 70mph, in 6th gear, here are your RPM's:

3.31 axle is turning 1530RPM
3.55 axle is turning 1641 RPM

With 18" rims and 275x70x18 tires, at 70mph in 6th gear:

3.31 axle is turning 1571RPM
3.55 axle is turning 1685 RPM

With 17" rims and 245x75x17 tires (dually):
3.55 axle is turning 1776 RPM
4.10 axle is turning 2051 RPM

Now, for 2017 peak torque is raised from the previous 1600RPM up to 1800RPM. Peak horsepower remains at 2800RPM. So you can see from the above information how some combinations are better for towing and some are better for economy.

 

Thanks, Trover.

Just thinking about the math you did gives me a headache. I usually only get as far as, "If two beers is good, then four must be twice as gooder......"

Ford moved the 6.7's torque curve up a little....... But I gotta wonder....... If it's max torque is 925 lb-ft @ 1,800 RPM as opposed to 860 lb-ft at 1,600 RPM......

Here's my dumas question for the day (I'm allowed one per day)

Is the engine producing less torque at 1,600 RPM than it was last year or about the same??

I can't find a dyno chart for anything on the 2017 6.7

 

The peak torque change has intrigued most of us here, Big Tow. The Cummins produces peak torque at 1700RPM; the Duramax produces peak torque at 1600RPM, and Ford produces peak torque at 1800. Does the Ford still produce 860lb-ft at 1600? Good question.
I don't know the answer, but I'd guess it does NOT.

 

The 18" option has 275/65/18 tires (at least my truck does). So at 70 mph the engine should be turning 1685 rpm (3.31) or 1810 (3.55) in 6th gear.

I used this calculator. Not sure about accuracy?

 

Do you have 20's.....?

The ones I've looked at online are 275/70/18 or 275/65/20

As always, I could be wrong

 

No, I have 18's.