Estimated MPG for Ford F350 Diesel w/373 rear end

[ANewCreation]

Hello everyone, I have a question. I am looking to buy a 2015 Ford F-350 Power-stroke with a 373 rear end and would like to know the estimated MPG difference between it and a 2015 F-350 Diesel with a 331 rear end? Trying to decide which rear end to get. Either is more than enough to tow my TT. Please base your estimate on normal highway driving while NOT towing anything. Thanks a bunch.

[Community Alumni]

My 2011 F-350 6.l diesel, SRW, gets 22 MPG on highway trips where we run at 55 MPH (90 km/h). Tapers off to about 17 at 70 MPH.

Towing our 38'/13,000 lb. 5th wheel at 55 MPH gives us a shade over 12 MPG.

[Russwolford]

That’s about what I get with the same setup.

[way2roll]

Not sure if you were aware but this topic is 11 years old. 

[cdlaine]

Outstanding. Thanks Niner. It took a couple re-reads but I
think I got it. I recall dad and his gear head buddy's referring to
taller gears back in the day... I envisioned a physical conformational
change in the gear mechanism.

Thanks for the shop talk.

Charles

[NinerBikes]

cdlaine wrote:
Wow... the Ford crew here has me officially jealous with these MPG numbers
being posted. My gasser typical... unloaded (around town) 10.5-11 MPG. Loaded
(flatlands 7.5-8 MPG).

Best I have ever tallied...
15 MPG (unloaded) on a long haul , flatlands, with cruise control at 55 mph.
Hats off to your diesel Ford numbers.

BTW... what does "taller gears" mean ???

Charles

A vehicle geared "tall" has gearing such that the motor loafs and turns less rpms at any given speed, while in overdrive. So if you are doing 60 mph, your motor turns 1800 rpms, instead of 2200 rpms. Important when driving a diesel as diesel motors, because of the way diesel fuel burns, are optimized for a sweet spot for maximum fuel efficiency of around 1500 rpm (think Cummins inline 6) in bigger motors, to 1900 in smaller 2 liter diesel motors (think Volkswagen SUV's and sedans, where each cylinder is .5L in displacement.)

The flame front of diesel fuel ignition moves very slow, while making power, such that higher rpms defeats getting a complete full burn of the fuel, the piston falls away on the ignition stroke faster than the fuel can burn, above a certain, usually max torque rpm figure. So you want to be running that motor as close to that max torque rpm, regardless of load, at almost all times. Having an 8 speed gearbox, or a 12 speed, allows this to occur more frequently at all driving speeds. With enough gears in a gear box, being geared tall or short should be a non issue, regardless of load you are towing, the transmission will select a lower gear to run if need be, for the amount of weight towed.

[cdlaine]

Wow... the Ford crew here has me officially jealous with these MPG numbers
being posted. My gasser typical... unloaded (around town) 10.5-11 MPG. Loaded
(flatlands 7.5-8 MPG).

Best I have ever tallied...
15 MPG (unloaded) on a long haul , flatlands, with cruise control at 55 mph.
Hats off to your diesel Ford numbers.

BTW... what does "taller gears" mean ???

Charles

[spud1957]

Ford's GCWR and max trailer weight are the same for both the 3:31 and 3:55.

[Bamaman11]

I've never heard many diesel truck owners (any brand) complaining of their truck not having the power to pull. The DEF trucks just have incredible torque.

Don't get too hung up on gear ratios. Variables are the final gear ratios of the transmission and the diameter/height of the tires on the truck.

If you have the choice, get the Ford with the lowest gear ratio available. If it comes just with a 3.31, rest assured that it's going to tow your load.

And for fuel mileage, just keep it below 2000 rpm's. That's the sweet spot for fuel usage.

[wintersun]

You will reduce towing capacity with the 3.31 gears and might actually hurt fuel economy as well. These engines produce peak torque at around 1600-1700 RPMs and work well with the 3.73 gears at highway speeds when towing. When not towing the 6-speed transmissions provide 2 overdrive gears that more than compensate for the lower gears.

The factor that most affects MPG is the driver. Fleet truck operators have drivers that average 6 MPG with the same rigs that other drivers manage to average 8 MPG over the course of a year. That is a 33% difference and it is widely known that there are "8 mpg drivers" on the road and it is how they drive that is very different.

I still find it difficult to understand why so many people are concerned about MPG before they buy a truck and then drive at 10-20 MPH over the speed limit and later complain about the lousy fuel economy and high price of diesel.

[GoPackGo]

"If you are towing at highway speeds you don't really want your truck in overdrive."

Well, this sure isn't true. Both 5th and 6th gears of my 2013 Ford F350 trans are OD, and I pull 14,000 pounds of 5th wheel all the time. I just put the trans in 'D', push the Tow/Haul button, and go.

Tim.

[4x4ord]

ANewCreation wrote:
I stand corrected! Ford does NOT offer a 373 on a SRW F-350 Diesel:( Since I don't want a dually I may have to look at Dodge. Again, thanks guys, at least I have some idea of what MPG is.

Ford doesn't put 3.73 rear ends in their srw trucks because it would be foolish to have a 3.73 rear end coupled to their 6r140 transmission and the powerstroke engine in a srw pickup. People don't realize how the 6 speed transmissions and high torque of the new engines work together to provide huge off the line torque. The new Ford with a 3.55 rear axle is capable putting of over 11,000 lbs of torque on the rear axle in low gear. A 2004 Cummins with 4.10 rear end would put 5600 lb ft of torque on the rear axle in low gear. At highway speeds when not towing a general rule is that the slower the engine turns the better your fuel economy will be. If you are towing at highway speeds you don't really want your truck in overdrive.

[Turtle_n_Peeps]

ANewCreation wrote:
I stand corrected! Ford does NOT offer a 373 on a SRW F-350 Diesel:( Since I don't want a dually I may have to look at Dodge. Again, thanks guys, at least I have some idea of what MPG is.

:h :h :h

[shelbyj]

Don't not buy a Ford because they don't offer 3.73 gears. Its all relative. Tranny gears play a part too.

[ANewCreation]

I stand corrected! Ford does NOT offer a 373 on a SRW F-350 Diesel:( Since I don't want a dually I may have to look at Dodge. Again, thanks guys, at least I have some idea of what MPG is.