MPG for 3.5 with 3.73 - Page 2 - Ford Transit USA Forum
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post #11 of 122 (permalink) Old 03-19-2015, 11:06:AM
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But a 13% savings on gas costs is nothing to sneeze at, all other favors being equal . . .

2015 XLT 150 Wagon, 3.5EB/3.31LS, 130" WB, low roof, 8-passenger, full trailer tow incl. brake controller, ordered 12/14, delivered 4/20.
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post #12 of 122 (permalink) Old 03-19-2015, 12:11:PM
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Originally Posted by PeterR View Post
But a 13% savings on gas costs is nothing to sneeze at, all other favors being equal . . .
And you rudely accused me of having issues with reading comprehension?

I said 13 percent difference in gearing, and that it wouldn't make much difference in efficiency. I wrote NOTHING about a 13% savings in fuel costs. In fact I'd state that's impossible in this case.

Sorry Peter, but if you hadn't been so rude and close minded towards learning anything new in engineering/technical field, I would gladly help you understand the difference. But you made it clear on two occasions you don't want to be bothered with facts.

Besides, it's not a simple subject to analyze, and with your confirmation bias towards everything 3.31 I'm not going to waste my time knowing you aren't going to change your mind anyway.
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post #13 of 122 (permalink) Old 03-19-2015, 12:27:PM
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But a 13% savings on gas costs is nothing to sneeze at, all other favors being equal . . .
That's 13% higher RPM in top gear, not gas consumption.

The biggest difference would be about 13% more energy being lost to internal engine / transmission friction due to the higher RPM. This difference should be quite small compared to the total used at highway speeds.

The engine may be VERY slightly more or less efficient at generating power (disregarding internal friction) at a 13% different engine speed.

Most of the consumption of the energy is unchanged:
Aerodynamic, vehicle rolling resistance, acceleration / hill-climbing.
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post #14 of 122 (permalink) Old 03-19-2015, 12:57:PM
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Sorry for the imprecision with the 13% figure -- the main point being that the 3.31 rear end is, in my personal opinion, going to yield better MPG than the 3.73, with all other factors being equal, at 65 MPH in 6th gear.

2015 XLT 150 Wagon, 3.5EB/3.31LS, 130" WB, low roof, 8-passenger, full trailer tow incl. brake controller, ordered 12/14, delivered 4/20.
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post #15 of 122 (permalink) Old 03-19-2015, 01:37:PM
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Originally Posted by VinceP View Post
That's 13% higher RPM in top gear, not gas consumption.

The biggest difference would be about 13% more energy being lost to internal engine / transmission friction due to the higher RPM. This difference should be quite small compared to the total used at highway speeds.

The engine may be VERY slightly more or less efficient at generating power (disregarding internal friction) at a 13% different engine speed.

Most of the consumption of the energy is unchanged:
Aerodynamic, vehicle rolling resistance, acceleration / hill-climbing.
Well written.

If we look at Brake Specific Fuel Consumption maps of various engines, there is typically a "maximum efficiency" area that would yield best fuel economy -- assuming everything is equal. As long as we stay in that area or zone, trading RPMs for torque or vice versa won't make much difference in MPGs.

On the other hand if gearing is made too tall for the required HP, the engine will be forced above the highest efficiency zone of the BSFC map, thereby wasting gasoline. Fortunately in today's computired world the transmission would likely shift down to improve efficiency.

I'm guessing you already knew that.
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post #16 of 122 (permalink) Old 03-19-2015, 02:18:PM
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Sorry for the imprecision with the 13% figure -- the main point being that the 3.31 rear end is, in my personal opinion, going to yield better MPG than the 3.73, with all other factors being equal, at 65 MPH in 6th gear.
Agreed. There will almost certainly be a small improvement in that situation.
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post #17 of 122 (permalink) Old 03-19-2015, 02:44:PM
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Well written.

If we look at Brake Specific Fuel Consumption maps of various engines, there is typically a "maximum efficiency" area that would yield best fuel economy -- assuming everything is equal. As long as we stay in that area or zone, trading RPMs for torque or vice versa won't make much difference in MPGs.

On the other hand if gearing is made too tall for the required HP, the engine will be forced above the highest efficiency zone of the BSFC map, thereby wasting gasoline. Fortunately in today's computired world the transmission would likely shift down to improve efficiency.

I'm guessing you already knew that.
Thanks.

I haven't seen a BSFC map for a long time; I would be curious to see one for a modern road-going production engine. I had assumed it probably wouldn't be dropping off an "efficiency cliff" for a 13% RPM / load difference.
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post #18 of 122 (permalink) Old 03-19-2015, 08:44:PM
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Fuel Mileage Questions....

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Thanks for the updates Nathan. Any chance of a side photo showing your rig with the trailer hooked up, for height comparisons, or a comment on how the medium roof van and trailer tops align? Nice work on the throttle and brakes IMO.

What was your previous tow vehicle if any, and are you happy with your MPG? If you had the time to order a new Transit, do you feel that the 3.31 rear end would work for your needs, or does the 3.73 give you a margin of power/gearing for the loads you have to move? I assume that most of your highway miles are in 6th gear -- what speed and revs. would you say are the norm?

If possible could you also give us an idea of your total van and trailer weights loaded?

Thanks again, and sorry for all the questions . . .

Peter



Previous tow vehicle..1996 suburban diesel. Anything over 2000lbs...constant downshifting..at max load 5500lbs box trailer(high top) could not maintain 55mph. Constant overheating problems. very unsafe.


MPG...with the suburban at 5500lbs(high top) trailer...7.5-8mpg DIESEL, towing my "lite" box trailer(1750lbs, aluminum)...12.5-13mpg DIESEL


With the transit..towing the 5500lb trailer....8-9mpg, my lite box trailer 12.5-14mpg. The transit doesn't even break a sweat with the lite trailer. The bigger trailer was a little more adventurous, but no problems keeping speed. The bigger trailer is a good bit taller....so that kills you in the wind.


IMO I think the tow rating are a bit on the cautious side....honestly think you COULD pull with the 3.31 without any problem....but didn't want to push it with warranty issues, etc.


If I were to buy(and warranty was not a concern)...3.31 is plenty. But ford says 3.31 is only good for 5400lbs in my config, 3.73 is 6800lbs.


But for 90% of people, 3.31 is plenty. I am pretty confident you would have no problems unless you are pushing 5000lbs constantly.


I will have to look, but think I am 2000-2250rpm most of the time.(non towing) With up to 3000lbs you don't feel a thing..6th gear 95% of the time. With the larger trailer it did kick down to 5th about 25% of the time on hills.


Overall I am VERY pleased. Sure, I would like another mpg or two...but don't think you can do much better unless you are in a super duty or something with the 6.7 diesel...then you pay the diesel premium.


Overall, my fuel mileage is up about 30-40% and no worries at all. Very calm and engine does not feel like it is struggling at all.


Will try to attach pics. P


PS--the Sweet Pea trailer is much larger/heavier. Look at how much it sticks up into the wind. The grey stealth trailer sits mostly behind the transit...
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Last edited by Nathan Green; 12-31-2015 at 01:09:AM.
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post #19 of 122 (permalink) Old 03-19-2015, 08:52:PM
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Originally Posted by VinceP View Post
Agreed. There will almost certainly be a small improvement in that situation.

Agreed, unless you are towing at max and need the extra safety margin..3.73 is overkill with the EB. I would without hesitation tow 5000lbs with the 3.31....but didn't want any question at all should there be a powertrain/warranty issue.


Nate
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post #20 of 122 (permalink) Old 03-19-2015, 09:28:PM
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Thanks Nathan for the detailed reports, much appreciated. I can see why the aluminum trailer gives such good MPG and how it tucks right in there.

2015 XLT 150 Wagon, 3.5EB/3.31LS, 130" WB, low roof, 8-passenger, full trailer tow incl. brake controller, ordered 12/14, delivered 4/20.
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