What rearend gear ratio to improve fuel mileage - Diesel Forum - TheDieselStop.com
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99 & up 7.3L Power Stroke Engine and Drivetrain Discussion of the 99 & up 7.3L Power Stroke diesel engine and drivetrain in the 1999-Up Super Duty trucks and Excursions. No gas engine discussion allowed except on transmissions and drivetrain that pertain to all models. Please confine discussion of topics in this forum to those items that are specific to the 7.3L Power Stroke engine.

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Old 10-09-2006, 10:53 PM   #1 (permalink)
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What rearend gear ratio to improve fuel mileage

OK all you gear ratio and fuel mileage gurus (SmokeyWren and Dave Whitmer and others).

Where I live we get to drive about 80 mph, which puts my rpms around 2300, not ideal for fuel mileage. I drop from around 19 mpg at 65 mph to 15-16 mpg at 80 mph.

Part One: Here is what I am thinking. If I go with a set of taller gears, what gear ratio do I need to allow me to drop my rpms below 2000 at 80 mph. I can do the math on part one, but part two is beyond my knowledge.

Part Two: I also tow a 30' tag camper, about 7,000 to 8,000 pounds. Currently, I pull at about 70 mph/2000 rpm, when it downshifts for a hill the rpms go up to 2500. What gear ratio could I pick so that my rpms without OD would be about 2000 at 70 mph. In other words, what taller gear ratio, would help me improve my regular 80 mph cruising rpms and let me use a lower gear (or lack of OD) similarly to my current 3.73s?

Now the really important question. What type of MPG improvements might I expect to see? Would it pay off in my lifetime? And more important, is this stupid idea?[img]/ubbthreads/images/graemlins/shrug.gif[/img][img]/ubbthreads/images/graemlins/shocked.gif[/img][img]/ubbthreads/images/graemlins/shrug.gif[/img]

Thanks,

steve
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Old 10-09-2006, 11:00 PM   #2 (permalink)
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Re: What rearend gear ratio to improve fuel mileage

It sounds to me like you have the 3.73s

Ive got the 4.10s, and I see 2k RPM at just 65 mph.

I beleive the only other gear offered was a 3.55 Which would drop your RPMs a bit more.

Only other option is to increase tire size. [img]/ubbthreads/images/graemlins/shrug.gif[/img]
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Old 10-10-2006, 12:01 AM   #3 (permalink)
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Re: What rearend gear ratio to improve fuel mileage

Get one of the gearvendors overdrive. That will drop your RPMs from over 2000 to less than 2000 at 80mph. They are expensive though.

wells
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Old 10-10-2006, 07:53 AM   #4 (permalink)
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Re: What rearend gear ratio to improve fuel mileage

I'm sure David will chime in, but I believe he's running 3.08's in the rear with 235 tires all the way around. BUT.... he also has a 6-speed. Big difference to consider there.
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Old 10-10-2006, 08:11 AM   #5 (permalink)
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Re: What rearend gear ratio to improve fuel mileage

[ QUOTE ]
If I go with a set of taller gears, what gear ratio do I need to allow me to drop my rpms below 2000 at 80 mph.

[/ QUOTE ]

Assuming your 2,300 RPM at 80 MPH is accurate, and you want 2,000 RPM at 80 MPH:

300/2300 = 13 percent

3.73 minus 13 percent = 3.24

[ QUOTE ]
Currently, I pull at about 70 mph/2000 rpm, when it downshifts for a hill the rpms go up to 2500. What gear ratio could I pick so that my rpms without OD would be about 2000 at 70 mph.

[/ QUOTE ]

OD is 0.71, so to have the same RPM without OD you'd have to reduce your rear end ratio number by 29 percent.

3.73 minus 29 percent = 2.65

[ QUOTE ]
In other words, what taller gear ratio, would help me improve my regular 80 mph cruising rpms and let me use a lower gear (or lack of OD) similarly to my current 3.73s?

[/ QUOTE ]

That's not the same question.

Ford offered a 3.55 rear end ratio in the OBS PSDs, and the folks that didn't tow much liked them okay. But the folks that bought their trucks to be used as a truck wished they had ordered the 4.10 rear end. (3.73 wasn't available back then).

Trying to guess at a number you'd be satisfied with is a gamble. I suspect you'd like 3.55 ratio okay. Stretching it out to a 3.08 ratio would be a real gamble - you'd love it when unloaded, but probably hate it with 8,000 pounds tied on the back end. But Dave Whitmer is the expert on this max MPG stuff. I think he runs a 3.08 rear end ratio and is still able to tow a trailer up the mountain.

The best way to have your cake and eat it too is to go with an auxiliary overdrive from Gear Vendors or U.S.Gear. Then you'd have your current setup for towing, but reduce the RPM about 20 percent when you kick in double overdrive. 2,300 minus 20 percent = 1,840 RPM at 80 MPH. Now them's some long legs. [img]/ubbthreads/images/graemlins/warmsmile.gif[/img]

[ QUOTE ]
Now the really important question. What type of MPG improvements might I expect to see? Would it pay off in my lifetime?

[/ QUOTE ]

Probably not. Engine RPM is only one piece of the puzzle. The killer for MPG is aerodynamic drag. You're trying to push a brick through the air at 80 MPH. Regardless of rear end ratio, engine RPM, or anything else, your fuel mileage is going to drop a bunch when you increase the speed through the air from 70 MPH to 80 MPH.

With my trailer that also grosses about 8,000 pounds, I get about 9 MPG at 73 MPH (2,150 RPM), but if I slow down to 62 MPH (1,800 RPM) I get 12 MPG. Almost all of that difference is the result of aero drag.

[ QUOTE ]
And more important, is this stupid idea?

[/ QUOTE ]

Not at all. You just have to realize that any change to rear end ratio is a compromise between towing power and unloaded cruising economy. You can't have both without adding another gear - which is what the auxiliary overdrive does.
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Old 10-10-2006, 10:06 AM   #6 (permalink)
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Re: What rearend gear ratio to improve fuel mileage

I went from 4.10 to 3.73, was an increase of about 1.5mpg depending on how I drive. This is mixed highway/behind stupid drivers who cant do 55 in a 55 driving. I also had upgraded my PCM with a PMT with the DP tune 80 HP program, so off stock I dont know what the actual increase will be. I did the gear reduction because I blew the rear end out from lack of lube and since I had to replace it all, I decided to go down, and my only real non repair cost was the front regear, which was about $500. So I may see a payoff in improved mileage, provided I can stop smoking people! [img]/ubbthreads/images/graemlins/laugh.gif[/img]
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Old 10-10-2006, 10:19 AM   #7 (permalink)
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Re: What rearend gear ratio to improve fuel mileage

[ QUOTE ]
[ QUOTE ]
Now the really important question. What type of MPG improvements might I expect to see? Would it pay off in my lifetime?

[/ QUOTE ]

Probably not. Engine RPM is only one piece of the puzzle. The killer for MPG is aerodynamic drag. You're trying to push a brick through the air at 80 MPH. Regardless of rear end ratio, engine RPM, or anything else, your fuel mileage is going to drop a bunch when you increase the speed through the air from 70 MPH to 80 MPH.

With my trailer that also grosses about 8,000 pounds, I get about 9 MPG at 73 MPH (2,150 RPM), but if I slow down to 62 MPH (1,800 RPM) I get 12 MPG. Almost all of that difference is the result of aero drag.



[/ QUOTE ]

Smokey has hit the nail on the head. Drag is the big killer. Drag increase with the square of speed.

[ QUOTE ]
Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Since power is the rate of doing work, exerting four times the force at twice the speed requires eight times the power.

[/ QUOTE ]

I doubt that you can have gearing pay for itself. Remember, you got a 4x4.
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Old 10-10-2006, 03:41 PM   #8 (permalink)
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Re: What rearend gear ratio to improve fuel mileage

Unless you do something dumb like me and not check the fliud level Then its a maintenance issue.
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Old 10-10-2006, 04:38 PM   #9 (permalink)
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Re: What rearend gear ratio to improve fuel mileage

You could put a US Gear or Gear Vendor overdrive on the truck. this allows you better fuel mileage when running without the trailer. My guess is 2-3 MPG better. When you are towing your Amish barn with wheels, you click the overdrive off.

An overdrive is salty - about $3500. Even at today's fuel prices count on 100,000 miles to recoup the investment.

If you do put an overdrive on, consult with one of the automatic transmission gurus for mods to let the automatic stay together. An overdrive gets you better MPG by slowing down the engine to reduce engine frictional HP. Problem is that this also slows down the torque converter and less fluid gets pumped through the cooler and the frail transmissions we have won't take that very long. I don't think much of an automatic if you never towed anything better than a motorcycle, much less the big load you have.
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Old 10-10-2006, 07:47 PM   #10 (permalink)
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Re: What rearend gear ratio to improve fuel mileage

Thanks for everyone's help on this.

Smokey, I was afraid I couldn't have my cake and eat it too. I was pretty sure I couldn't be lucky enough for the OD ratio and the desired rpm shift be comparable with what I was thinking.

CoolCanook, it is nice to know the aero drag is a square relationship, makes things more clear to me.

I kind of expected that the miles of service required to pay for any modifications would be way out there.

Based on what Dave says about the tranny (193,000 miles on it, unless the previous owner had one put in), maybe I should continue to save my pennies for a BST first.

I am still impressed that I can get 15 - 16 mph pushing the largest production SUV in the world down the road at 80 mph, with my family of six and 400 pounds of luggage in back. [img]/ubbthreads/images/graemlins/cool.gif[/img] [img]/ubbthreads/images/graemlins/cool.gif[/img] [img]/ubbthreads/images/graemlins/cool.gif[/img]

Thanks for everyone's help, this site is a great help.

steve
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Old 10-10-2006, 08:25 PM   #11 (permalink)
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Re: What rearend gear ratio to improve fuel mileage

[ QUOTE ]
Smokey has hit the nail on the head. Drag is the big killer. Drag increase with the square of speed.

[/ QUOTE ]

I thought it was a cubed law; drag increases with the cube (3rd power) of speed?

I think my dad told me that (he was a chemical engineer, not an aero). I'm an electrical engineer, so what do I know???

Andy
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Old 10-10-2006, 09:02 PM   #12 (permalink)
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Re: What rearend gear ratio to improve fuel mileage

Running 80mph in your truck is going to hurt the mileage no matter what gears you throw under it. It takes "X" amount of horsepower to move your 8,000# brick through the wind at 80mph, it takes "X" amount of fuel to build that amount of power from your PSD regardless of rpm (internal parisitic drag is not even an issue compared to the aerodynamic drag)


Don't waste your time.

Buy a Honda to drive when you're not towing your trailer, or.....SLOW DOWN!!!
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Old 10-10-2006, 10:08 PM   #13 (permalink)
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Re: What rearend gear ratio to improve fuel mileage

444turbodiesel,

Here is my choice:

80 mph & 15-16 mpg -->32 hours of travel on good roads with four kids in the back.
Or, 60 mph & 19 mpg --> 40 hours of travel on good roads with four kids in the back...

You be the judge.

So far, we have decided that driving fast is better for the family.[img]/ubbthreads/images/graemlins/grin.gif[/img]

steve
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Old 10-10-2006, 10:45 PM   #14 (permalink)
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Re: What rearend gear ratio to improve fuel mileage

Question: If my rpms are reduced, and moved into a stronger part of the power curve, wouldn't I develop the required horsepower with less fuel consumption? More efficient development of horsepower?
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Old 10-11-2006, 08:17 AM   #15 (permalink)
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Re: What rearend gear ratio to improve fuel mileage

[ QUOTE ]
Question: If my rpms are reduced, and moved into a stronger part of the power curve, wouldn't I develop the required horsepower with less fuel consumption? More efficient development of horsepower?

[/ QUOTE ]

No.

Look at the torque curve and the horsepower curve.

The 250 horse 7.3L makes both enough torque and enough HP to tote the mail between about 2,000 and 2,600 RPM. Less than about 2,000 RPM, and especially below 1,900 RPM, you don't make enough horses. More than about 2,600 RPM and the torque curve drops like a rock and the HP curve head down hill.

So anything between 2,000 and 2,600 is the "sweet spot".

With a good towing tune, you can drop the RPM to around 1,750 and still make enough horses to tow 8,000 pounds. So that's why my rig is a hot-rod towing machine. I usually cruise at 1,800 RPM and about 62 MPH when towing. Without the hot rodding, I had to maintain a minimum of at least 1,900 RPM.
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