3.31 vs. 3.55 locking rear end & MPG? - Ford Powerstroke Diesel Forum
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Old 02-13-2011, 03:46 PM
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3.31 vs. 3.55 locking rear end & MPG?

I'm thinking of just ordering a 2011 since I can't find exactly what I want....

My question then is: Will getting 3.31's instead of the 3.55's (which are more common) improve my fuel economy (especially empty on the highway)? If so, by how much?

I'll have 35" MT's on 18" wheels. TT is about 7.5k dry. Current boat is only 5,750, but the boat we want in a year or so will be 13k loaded up.
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Old 02-13-2011, 03:56 PM
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3.31s will get better mileage. I don't know how much. Hauling wise, you'll be fine with either setup. 13k isn't too bad. All of the Superduties are designed to haul so you'll be fine with whatever setup you choose. If you were hauling much more than 13k, I'd go with 3.55s or 3.73s.
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Old 02-13-2011, 04:10 PM
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If mpg is your goal, buy a truck with 3.31's put 20's on it after you get it home. Make sure the 20's have a very low rolling resistence number,or atleast go as low as your comfortable with. Lower rolling reisistence equals less traction in sticky situations. There are trade offs for everything. You need to figure out how much traction you need and how much mpg your willing to give up.
Good Luck,
Supershooter
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Old 02-13-2011, 04:13 PM
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Definitely sticking with my 35" MT's. I need the traction in the mud when hunting and on the beach. I know they kill mpg's, but that aside, trying to confirm that I can get away with 3.31 instead of 3.55.
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Old 02-13-2011, 04:18 PM
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Go with 3.55 if you are going with taller tires when you get it home and it will be closer to a 3.31 with factory tires
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Old 02-14-2011, 04:59 PM
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I have pretty much made up my mind on an F250 XLT 4x4 Supercab, and have been wrestling with the tires and gears details, also. I test drove my first 6.7 Saturday, a Lariat crew cab with 18" tires and 3.55's, with about 50 miles on it. Not much of a test, I put about 30 secondary road miles on it, left the dealer showing about 10 mpg on the dash and showed 16 on the screen when I got back, if it can be believed. Speeds varied , but mostly 60 to 70., about 2 miles of in town. Mathmatically, the difference between the 17" tires circumference with 3.31 gears and 18" tires circumference with 3.55's works out about even. I haven't seen one with 20's. Good luck with your decision, I'm still working on mine. Pete
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Old 02-15-2011, 02:13 AM
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i have been round and round between this and color for the last 3-4 months. I am going to attach a chart i made showing different stock tire sizes and what the effect would be if you up sized the tires. it will show your effective gear ratio and final drive ratio. I originally did this to see what kind of grunt you will have off the line.

I recently conversed with a guy from Canada on another site who hand calculated 19 hwy with factory tires and 17 hwy with 35's and 3.31 gears. Seeing as how he is not in a flat area i think i could still get good mpg since i live in Houston. It makes you wonder though would the truck do better with 3.55 and a bigger tire lowering your gear ratio and having less load on the motor vs a 3.31 and lowering that ratio and having more load on the motor?

Then i sit back and look at what the guy from Canada is seeing and realize with his setup i would see a minimal of 30% improvement hwy unloaded and close to 50% more hwy mpg loaded than my current truck.
Attached Files
File Type: pdf 2011 Ford Super duty final drive ratio comparison.pdf (140.6 KB, 862 views)
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Old 02-15-2011, 05:08 AM
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So the truck I'm looking at has 275/70R18 tires on it with 3.31 gears. So the new effective drive ratio would be 3.14:1.

My understanding is that this would increase my MPG on the highway (and general cruising) by lowering the RPMs at speed and decrease my off the line performance, which could impact 0-60 times and getting a heavy load moving.

I don't see towing 7k lbs or even 13k lbs being a problem with the 3.31's. Am I wrong?
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Old 02-15-2011, 01:17 PM
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you thinking is correct but there is a fine line here. the rpm difference between the same tire with different gears is about 100. well the problem is we do not know if it takes more fuel to turn those extra rpms with the 3.55 which will have less load because of the gearing or if it uses more fuel with the 3.31 at a lower rpm under more load.

just comparing what people have reported with stock trucks, the few 3.31 guys have had better economy both unloaded and towing. if you look at the chart as far as low end grunt you will notice that the configuration you speak of actually has more 1st gear grunt than the 2010 ford and the current dmax and cummins.

I think i am just going to be the Ginny pig and go with the 3.31's

If any of my thinking is skewed someone please correct me, I am just a construction worker with a calculator.
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Old 02-15-2011, 05:15 PM
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sleepersilverado, I afraid that there are more factors that affect fuel ecomony than tire size, gear ratio, final transmission grear ratio. Factors such as rolling resistence also greatly affect fuel econom. Motors are also a key to fuel effeciency, as we all know different tunes from different after market companies affect our fuel economy. Ford has their way to tune a motor but, they have to follow emissions standards. I'm not sure we will find answer we are all looking so intently for. I'm just as interested in finding the answer as everybody else. If all I was interested in was fuel economy I'd buy the truck with the highest gear ratio, put tall tires on with very low rolling resistence, and get a programmer that makes the motor run at the place on the torque and hp curve where fuel economy bliss is found. Good Luck All- I'll be watching
Supershooter

Last edited by Supershooter; 02-15-2011 at 05:19 PM.
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