F350 Rear Axle Problem, Anyone Have The New 3:55 Gear Ratio Yet?

[72GTS]

My 2008 F350 "Tow Boss" has the mandatory 4:30's. Been hearing a loud and harsh clunking going into and out of park, reverse, and drive. Been getting worse. I have just 3900 miles on the truck, never towed anything more than a 7000 pound single car hauler yet. Anyhow took it to dealer today, and what I was afraid of was the case; gears are worn badly, the drive shaft makes a full half turn moving trans from drive to reverse, the bang is from the differential.
I am wondering if anyone has the fuel mileage difference, if there is any, with the 3:55 gears. I will have them put them in the back differential under warranty, and I'll pay for the front ones to be changed if there is a difference in mileage that is 2 or 3 mpg. I never will tow more than 15000 pounds , truck just had everything I wanted on it, so bought it off the lot.
But fuel was about 2.30 a gallon then! I drive mostly empty, around town and average 10.2 mpg now. Occasionally tow a car on a trailer to a show.
Any thoughts on ratio change would be appreciated. It goes in to be repaired Tuesday.

[RatherBNtheWoods]

Maybe you would want the 3.73's to tow 15K lbs? I have 4.10 and consider them to be a shade too low.

[Gregdg1971]

A 3.55 axle ratio will hurt your in town fuel economy ... you will not be running the engine at a high enough speed in normal city driving (i.e <45 MPH) to lock the torque converter in top gear and will take a fuel hit. The 3.55 ratio will help your highway fuel economy as you will be turning at lower RPMs at freeway speeds. Also, if you have a 4.30 rear end with dual rear wheels you have a Dana 80 axle ... the 3.55 ratio is a 10.5" Ford single rear wheel axle ... I don't believe you can get a 3.55 ratio for the Dana 80.

[therabbittree]

3.55 is avail for dana 80's dodges had them stock...

[jasonmt]

Originally Posted by Gregdg1971

I don't believe you can get a 3.55 ratio for the Dana 80.

OEM Dana 80 gear sets fo the 80 range from 3.31 to 5.38 but you are correct in that 3.55's are not available for the 80, you have to settle for 3.54's (46-13).

I would think that having a dealer put a 3.54 gear set in a tow boss truck that came with 4.30's as warranty work would be pretty slim, notwithstanding if there would even be 3.54's for the D80 available in Fords system...

[Stymie]

for the cost of parts and labor on the front, its not going to be worth the small gain in MPG

[ndmtnsledder]

I have a Job3 08 F250 4x4 shortbox crew cab with 16k miles now. I have the 3:55 gears and they work great for me. I tow misc stuff for the farm usually a 10k# loader and trailer combo, or 5k# with sleds and gear in my 27ft enclosed. I had been getting by with my F150 but just wanted to be able to do the same loads better. Anyway no towing problems at all with my loads handles them like a champ. Most of my driving is rural highway or interstate so speeds between 60 and 80 I average an honest 14mpg I'm not light on the pedal or acceleration. At 65mph with no headwind I get 16mpg pretty easy if I behave myself. For my use I'd even consider going up to the higher gear ratio next time. Towing power compared to my brothers 00 F250 7.3L with 3:73s (I'm guessing is whats in it anyway) is still better with the extra power of the 6.4L

[wingnut60]

How about a GearVendors overdrive? Do they make one for the 6.4?

[senix]

I would opt for the 3.73's no less. Or stick with what the truck came with.

[vntperformance]

Originally Posted by senix

I would opt for the 3.73's no less. Or stick with what the truck came with.

are there 08-up drw's that are offered w/3.73s?

[senix]

Originally Posted by vntperformance

are there 08-up drw's that are offered w/3.73s?

I think so :
Build and Price

I did a quick build and looks like it.

[Greyman]

I saw a gear vendors on Seattle craigslist for an 08 superduty, new in box, $2000.