Anyone running 35" or 37" tires with 4.10 or 4.30 gears? - Diesel Forum - TheDieselStop.com
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6.4L Upgrades and Aftermarket - Engine Upgrading or adding OEM or aftermarket equipment to your 2008-Up Super Duty truck with 6.4L Power Stroke diesel engine. Please confine discussion of topics in this forum to those items that are specific to the 6.4L Power Stroke engine.

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Old 01-24-2008, 10:04 AM   #1 (permalink)
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Anyone running 35" or 37" tires with 4.10 or 4.30 gears?

I'm running 35" tires with a Donahoe Racing 4.5" lift. My truck is a F250 Lariat, 6.4, CC, SB, SRW with 3.73's. My truck feels down on power and my city and towing mileage have suffered significantly (~8 in the city and ~7 towing 8k#). I've done all my research on re-gearing to either 4.10s or 4.30's, but before pulling the trigger wanted to hear from anyone that is running a larger tire and has re-geared.

Any input on how performance and gas mileage is post re-gear?
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Old 01-24-2008, 11:37 AM   #2 (permalink)
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wow, that mileage is terrible. My wife is seeing 11 in town with her 37's, although towing sucks nuts. Im going to subscribe to this thread, because i am also interested
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Old 01-24-2008, 01:20 PM   #3 (permalink)
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This is one that confuses me. I'm not sure about now, but I do know that several years ago, a guy could buy a Chevy 3/4 ton pickup with a diesel and 4.10 gears. Ford won't offer it, and that doesn't make ANY sense to me. Why would they make it impossible to order the gear you want?
I do have a couple of theories-
1- the engineers at Ford do not think that the driveline components under an SRW truck will handle the torque that the diesel motor puts out with a lower gear in it. You may want to look into this closely, because installing a different gear than stock may have warranty ramifications.

2- The SRW trucks have a Sterling 10.5 rear, and the duallys have Dana 80's, and, in the case of 450's and 550's, an even bigger Dana. You may want to look closely at this as well, because there is a possibility that the rear end case for the Sterling may not accept a bigger gear. The gear upgrade could be alot more complicated and expensive than you think. Just make sure before you buy new gears.
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Old 01-24-2008, 01:48 PM   #4 (permalink)
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This is one that confuses me. I'm not sure about now, but I do know that several years ago, a guy could buy a Chevy 3/4 ton pickup with a diesel and 4.10 gears. Ford won't offer it, and that doesn't make ANY sense to me. Why would they make it impossible to order the gear you want?
I do have a couple of theories-
1- the engineers at Ford do not think that the driveline components under an SRW truck will handle the torque that the diesel motor puts out with a lower gear in it. You may want to look into this closely, because installing a different gear than stock may have warranty ramifications.

2- The SRW trucks have a Sterling 10.5 rear, and the duallys have Dana 80's, and, in the case of 450's and 550's, an even bigger Dana. You may want to look closely at this as well, because there is a possibility that the rear end case for the Sterling may not accept a bigger gear. The gear upgrade could be alot more complicated and expensive than you think. Just make sure before you buy new gears.
1. - I've talked with the Diesel tech and service advisor at the local dealer I deal with and they said no problem with the warranty. They will actually do the gear install.

2. - You are correct that the rear sterling is a 10.5" ring gear. The install isn't difficult, but does require that the carrier is changed to accommodate 3.91 and up gears. The 3.73 carrier only handles 3.73 and down gears. The carriers are only about $80. The install front and rear parts and labor is $1300.
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Old 01-24-2008, 05:16 PM   #5 (permalink)
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the lack of gear is a fuel mileage issue. it has nothing to with the torque. the older diesels(OBS PSD) had 4.10's Ford is even going away from 3.73's and is going to 3.55's for mileage.
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Old 01-24-2008, 05:50 PM   #6 (permalink)
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1. - I've talked with the Diesel tech and service advisor at the local dealer I deal with and they said no problem with the warranty. They will actually do the gear install.

2. - You are correct that the rear sterling is a 10.5" ring gear. The install isn't difficult, but does require that the carrier is changed to accommodate 3.91 and up gears. The 3.73 carrier only handles 3.73 and down gears. The carriers are only about $80. The install front and rear parts and labor is $1300.
I had 35's with 3.73s and gas mileage was ok, but pulling the 40' toyhauler really caused the truck to shift a lot from 4th to 3rd

Upgraded to 4.10s and gas mileage was hardly affected (theoretically should have been worse), but pulled much better.

Lifted the truck to 8" with 37s with 4.10s and now I am back to square one.

My thoughts overall, if you are looking for gas mileage improvements don't waste the money. If you are towing absolutely do it. Just my 2cents.
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Old 01-24-2008, 07:32 PM   #7 (permalink)
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I had 35's with 3.73s and gas mileage was ok, but pulling the 40' toyhauler really caused the truck to shift a lot from 4th to 3rd

Upgraded to 4.10s and gas mileage was hardly affected (theoretically should have been worse), but pulled much better.

Lifted the truck to 8" with 37s with 4.10s and now I am back to square one.

My thoughts overall, if you are looking for gas mileage improvements don't waste the money. If you are towing absolutely do it. Just my 2cents.
This is exactly the information and experience I was hoping to see posted. I tow a car hauler and rock crawler from NorCal to SoCal and up in the Tahoe Area. I'm doing this mainly for pulling improvements. Thanks for posting!
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Old 01-24-2008, 09:24 PM   #8 (permalink)
 
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IMO if you want fuel mileage empty or towing get a 7.3 a 6.4 will never get it. Its to late to worrie about MPG so change your gears to 4.56 , that will really help with towing.

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Old 01-24-2008, 10:16 PM   #9 (permalink)
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IMO if you want fuel mileage empty or towing get a 7.3 a 6.4 will never get it. Its to late to worrie about MPG so change your gears to 4.56 , that will really help with towing.

Cary


I am not so sure whether I would run 4.56s with 35s . You will be revving pretty high under load or empty. Keep in mind that I my avg speed is higher than posted speed limit most of the time so it matters to me. Just my 2 cents
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Old 01-24-2008, 11:05 PM   #10 (permalink)
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I spent quite a lot of time researching gearing for my 6.0 PS Super Duty and even tried two different sets. I run 37" Toyos and innitally went with 4.56's which made the engine run to high of rpm's on the highway. The engine was above its comfortable cruising rpm. In short I hated them. I then went to 4.30's and they are perfect!!

If you are going to only tow a big trailer at lower highway speeds or on a lot of hills, then consider the 4.56's. I like to cruise at 75 to 85 and the lower gearing was not fun. For 35's, I would run 4.11's. Something to also keep in mind is that all the gearing formulas are based on a gas engines power curve and cruising rpm's. A formula that gives a gearing of 4.56 after a tire change is for a gas engine and should be adjusted down a step for a diesel application (i.e. to 4.11's.)

Mileage for my truck is 12 - 13 town at 5200 elevation and 16 - 18 highway. Best has been 22 on flat highway at sea level and cruise on 70 in CA.
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Old 01-24-2008, 11:46 PM   #11 (permalink)
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I'm running 35" tires with a Donahoe Racing 4.5" lift. My truck is a F250 Lariat, 6.4, CC, SB, SRW with 3.73's. My truck feels down on power and my city and towing mileage have suffered significantly (~8 in the city and ~7 towing 8k#). I've done all my research on re-gearing to either 4.10s or 4.30's, but before pulling the trigger wanted to hear from anyone that is running a larger tire and has re-geared.

Any input on how performance and gas mileage is post re-gear?

my truck came with the 20' on it witch comes with 34' tires on it and has plenty of power and olny going up 1' is not going to make much difference so it makes me wounder why your truck feals that way.
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Old 01-25-2008, 01:08 AM   #12 (permalink)
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my truck came with the 20' on it witch comes with 34' tires on it and has plenty of power and olny going up 1' is not going to make much difference so it makes me wounder why your truck feals that way.
But I'm not really going up just 1" in diameter. The standard tire size is 31.6" (265/70-17) and the 3.73 gears works very well with that size tire. So I'm really going up ~3-3.5" in diameter over the stock tire size. The 34" tires are an option that Ford offers on a lot of their trucks, but they do not compensate for the larger tire by adjusting the gear ratio to match. Increasing the tire diameter (and adding more rolling mass) will decrease the gear ratio from a stock 3.73 to a lower ratio (in my case 3.41). Once you go from a 31" to a 35" tire your effective gear ratio drops considerably which makes your truck work harder under load and at slower speeds. In other words, you need more throttle to get the truck moving the lower the gear ratio. It's kind of hard to explain, but unless you've driven a truck with stock tires, then upsized them you wouldn't think your truck is underpowered with the 34" tires.
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Old 01-25-2008, 01:58 PM   #13 (permalink)
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Thanks for all the input. I'm going to install 4.30's next week. I'll update the thread after I get a few miles on them.
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