Change out rear end? - Ford Powerstroke Diesel Forum
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post #1 of 12 Old 08-19-2010, 09:24 AM Thread Starter
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Change out rear end?

My truck currently has a 3.73 rear end in it. Fuel milage is terrible, especially with 35" tires. I only get about 10.5-12mpg with mixed driving. That is running the Spartan 275(TCM). How much trouble is it to switch out the rear end? What gear is recommended for better milage? I don't pull loads that are extreme heavy, but I do pull a three horse slant with living quarters. What can I do for better milage?

'08 F250 Crew Cab King Ranch 6.4L (White)
4" Zone Offroad Lift
Spartan Tuned
Spartan TCM
DPF/CAT Delete
EGR Delete
ARP Head Studs
S&B
MBRP
35x12.50x22 TOYO M/T
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post #2 of 12 Old 08-19-2010, 09:32 AM Thread Starter
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I guess the front would have to be changed too.

'08 F250 Crew Cab King Ranch 6.4L (White)
4" Zone Offroad Lift
Spartan Tuned
Spartan TCM
DPF/CAT Delete
EGR Delete
ARP Head Studs
S&B
MBRP
35x12.50x22 TOYO M/T
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post #3 of 12 Old 08-19-2010, 09:36 AM
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Yes, the front will have to be changed too. I wouldnt go lower than 3.55

Typical cummins owner.

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post #4 of 12 Old 08-19-2010, 09:39 AM
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If you are going to regear you need to do both front and rear - not sure on R&P cost for these trucks but assume 500 each axle with rebuild kit and another 400 or so for the install - so you are looking at probably 1800 - 2000 for the job. You could go down to 3.55's but how long would it take you to recoup the cost in fuel savings?

2011 F250 CCSB KR 4x4 FX4
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post #5 of 12 Old 08-19-2010, 09:40 AM Thread Starter
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I was thinking 3.55 is what I want. How much $ and or trouble would something like this cost? It will pay for itself over time...

'08 F250 Crew Cab King Ranch 6.4L (White)
4" Zone Offroad Lift
Spartan Tuned
Spartan TCM
DPF/CAT Delete
EGR Delete
ARP Head Studs
S&B
MBRP
35x12.50x22 TOYO M/T
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post #6 of 12 Old 08-19-2010, 09:42 AM Thread Starter
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I probably spend at least $400/ month in fuel. Probably more around $550.

'08 F250 Crew Cab King Ranch 6.4L (White)
4" Zone Offroad Lift
Spartan Tuned
Spartan TCM
DPF/CAT Delete
EGR Delete
ARP Head Studs
S&B
MBRP
35x12.50x22 TOYO M/T

Last edited by Down2Ride; 08-19-2010 at 11:19 AM. Reason: Text change
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post #7 of 12 Old 08-19-2010, 06:36 PM
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My guess is that 3.73 is your best choice. Running 3.55's is going to hurt performance, and probably won't increase mileage more than 0.5 mpg. Best MPG increase you can get is to cut your highway speed back, with the lift and increased drag - definitely keep it under 65 MPH.

Greg
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post #8 of 12 Old 08-19-2010, 06:58 PM
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What you need is to go higher, the taller tire's are lowering your overall ratio. This lowers your rpm's and takes away performance & mileage. Takes your engine out of the performance sweet spot, has to work harder. Maybe someone can explain it better, but you need to go up not down, expecially for towing or go back to smaller tires.
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post #9 of 12 Old 08-19-2010, 09:08 PM Thread Starter
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I'll probably end up keeping it like it is. I just hear all these people talking about 15-16mpg around town...stock. Hell, I wasn't getting but about 9. That's with 18" wheels and 275/70/18 Terra Grapplers. I put the Spartan on it with the 275 and got around 14mpg. Now with the 35s and 20s, I'm back around 10-11mpg.

'08 F250 Crew Cab King Ranch 6.4L (White)
4" Zone Offroad Lift
Spartan Tuned
Spartan TCM
DPF/CAT Delete
EGR Delete
ARP Head Studs
S&B
MBRP
35x12.50x22 TOYO M/T
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post #10 of 12 Old 08-19-2010, 09:10 PM Thread Starter
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All these #s hand calculated. I know it will be a little off due to the bigger tires, but not enough to make up for 4mpg, right?

'08 F250 Crew Cab King Ranch 6.4L (White)
4" Zone Offroad Lift
Spartan Tuned
Spartan TCM
DPF/CAT Delete
EGR Delete
ARP Head Studs
S&B
MBRP
35x12.50x22 TOYO M/T
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