Gear Calculations
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New User
Joined: May 2011
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Gear Calculations
Howdy All! Just Joined.
Im looking at new diesel 2011 f250's. I have read many of the threads regarding read end ratio. I see many posts that say the effect on hwy mpg is negligable between 3.31 and 3.55 ratios. I used the calculator at grimmjeeper.com and came up with these numbers assuming stock 265/70/17 tires.
@70 mph
3.31 = 1650rpm
3.55 = 1770rpms
@1750 rpms
3.31 = 74mph
3.55 = 69mph
it seems that the 5mph difference at equivalent speeds would lead to significant mileage differences. or at equal speed the 120rpms would effect economy.
Also - at some point i will end up with 34ish " tires maybe 285/75/17. when those numbers are entered. I get 1542 rpms at 70 mph with 3.31. That seems ideal considering the power band of this motor. I will tow heavy (15000 lbs) several times a year for short (100 mile) trips. Dropping back to 5th in this config will give 1838rpms at 65 mph which also seems pretty good.
I am obsessing over this a bit since i tend to keep my trucks for a very long time. Almost 13 years on my current 98 dodge. Just don't want to make a mistake.
Thoughts???
Im looking at new diesel 2011 f250's. I have read many of the threads regarding read end ratio. I see many posts that say the effect on hwy mpg is negligable between 3.31 and 3.55 ratios. I used the calculator at grimmjeeper.com and came up with these numbers assuming stock 265/70/17 tires.
@70 mph
3.31 = 1650rpm
3.55 = 1770rpms
@1750 rpms
3.31 = 74mph
3.55 = 69mph
it seems that the 5mph difference at equivalent speeds would lead to significant mileage differences. or at equal speed the 120rpms would effect economy.
Also - at some point i will end up with 34ish " tires maybe 285/75/17. when those numbers are entered. I get 1542 rpms at 70 mph with 3.31. That seems ideal considering the power band of this motor. I will tow heavy (15000 lbs) several times a year for short (100 mile) trips. Dropping back to 5th in this config will give 1838rpms at 65 mph which also seems pretty good.
I am obsessing over this a bit since i tend to keep my trucks for a very long time. Almost 13 years on my current 98 dodge. Just don't want to make a mistake.
Thoughts???
Posting Guru
Joined: Aug 2004
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From: Upstate, NY
The much larger issue for MPG will be the aerodynamic drag (C of D) differences at speed. Aerodynamic drag increases with the square of speed so you can expect 60% + of your energy to be consumed overcoming that. At 45 mph more than 50% of the resistance will be caused by aerodynamic drag. The best way to improve MPG with a vehicle that has a high C of D is to slow down. 62 MPH seems to be about as good as it gets for optimal MPG at highway speeds based on years of people comparing driving speeds to fuel economy. Obviously, terrain and weather also come into play, but generally speaking I believe this to be true.
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New User
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True - and im no engineer, but Im looking at equivalent vehicles (same aerodynamics) with different drive ratios and wondering what will be better for my needs. I live out of town and drive at freeway speeds mostly.
Possibly the argument could be made that 3.31 vs 3.55 it takes the same amount of work to hold 70mph. thus the only differences will be in what little friction delta there is between the different rpm settings. So only negligible fuel differences. IDK
Possibly the argument could be made that 3.31 vs 3.55 it takes the same amount of work to hold 70mph. thus the only differences will be in what little friction delta there is between the different rpm settings. So only negligible fuel differences. IDK
Senior User
Joined: Apr 2011
Posts: 115
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From: So Cal
I think most of the talk was about the difference between the gear ratios because you can't order the 3.31 gears with the 20" rims. The 20" rims are only available with 3.55 gears. Actually it looks like you can't get the 3.55 axle ratio without the 20" rims also.
Posting Guru
Joined: Oct 2010
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That's why many of us order our trucks. These vehicles are pretty expensive. Ordering will allow you to get what you want and not have to settle on something you didn't.
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New User
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is it getting too late to order a 2011? when do they start making 2012's? im sure the 2012 models wont have the good rebates for awhile.
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Tuned
Joined: Mar 2011
Posts: 265
Likes: 54
I too obsessed about the axle ratio when I was searching for my truck. I was also under the gun as I wanted to take advantage of the Ford purchase incentives ($5,000) before they expired. Anyway, I was set on the 3:31 ratio and looked far and wide in the SE MI area. Found a couple and the truck I selected (350 XLT, CC, SB, FX4) had the 18" wheel option. At 70MPH indicated on my GPS, the installed ScanGage indicates ~1604RPM. Love it. 18-20 MPG around town, and 19 at 70mph. With the torque this engine produces, I notice no drivability issues. I don't tow anything substantial in weight so I can't comment on towability, maybes other can. I weighed my truck at a local landscaper and it checked in at 8,010lbs empty with me in it. I am amazed how fast this truck accelerates, even with the tall gear. It is a sleeper.
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