Job 3 F-250 and F-350 6.4L trucks are equipped with a 3.55 rear axle as standard equipment.

I have a job 1 F-350 truck with a 3.73 rear axle and been thinking of changing the rear end to 3.55 I understand that many of the parts are re-usuable in the switch so cost could be about $1000 for axle set and labor. I have 18 inch tires

Questions are:
1. What fuel mileage improvement should I expect to help pay for the switch?
2. Will the switch effect my Ford warranty if I use Ford parts and shop labor?
3. My truck is now 11300 lbs GVW rating with 3.73 axle. Will the 3.55 axle reduce the GVW or will it remain the same?
4. I tow a 10600 lbs 5th wheel trailer. Will the 3.55 rear axle effect the towing performance?
5. Can anyone give me an estimate on the cost to do the switch?
6. If you think it is not worth the cost and effort pleae say so.

 

dont do it...3.55 to 3.73 will never get back the investment in fuel mileage.

 

I've been thinking about doing this with the F-450 I have. It has the 4.88 and I was thinking about a 4.10...I need over the road mileage...

 

It will be interesting to hear what the Job 3 owners have to say about their gas mileage with the 3.55. If the mpg gains are substantial, it may be worth it to make the switch as a long term investment.
But a thousand bucks is a lot of dinero.

 

Not worth it, not a big enough ratio difference to warrant the expense. You can get the same result by changing your tires.

 

Ya on the other hand, a taller tire is also more rotating mass and which can hurt fuel economy.

 

Pilot,
Also, keep in mind if your truck is 4WD you will need to do the front rear or else you may (will likely) grenade your transfer case.

CM

 

CM:

Never thought about the transfer case being matched to the rear axle. I will be at the dealership tomorrow to pick up my job 2 front air dam and the grille cover so I will bring up your concern with them. Thanks for the heads up.

 

You won't grenade a 271/273, you'll just put more wear on the chain. It also won't engage on the fly, you'd have to shift it on the stop. If you only use the 4x4 off-road it wouldn't be the end of the world but I wouldn't recommend it. Crow hop on turning will be worse since the mismatched ratios will make the rear axle turn faster when it needs to turn slower in a turn. You'd have to drive it a long way on dry pavement with the mismatched ratios to blow it up though.

 

You are corrct Brockway. However, the T/C chain will eventually [and prematurely] fail due to the mismatched gears.

Chris

 

and if yu ever get it into 4wd, you would probably have to jack one end of the truck up to get it out of 4wd. it sure wuld be fun to see it trying to go down the road with the front wheels spinning and you trying to control it jusd begore the u-joints or something else granades,,,hehehe. 3.55 s would be kind of a tall gear to pull. if it ever got into od it would not have much exceeration compared to 4.10s.

 

didnt brockway, the original cortland beater go out of business a few yeas ago? now there was true junk.

 

listen uppilot, you MUST change both differentual gears in a 4wd. you cannot have 355 rears and 373 front gears. it will be driveline shot for pilot shot if he trys this. listen to the old timers who were running the old knuckle front drives before you were born. the transfer box needs no changes but you must match the drive axle ratios.

 

It would seem to be common sense that if you change the rear diff you would have to change the front diff also. There would be no other way.

 

This is bad advice.
I have seen chains break because of mismatched ratios.
Also broken output yokes, inner axles and front outputs bound so bad you had to jack the truck up to beat it out of 4wd.
And the hopping you feal turning has nothing to do with the gear ratio.
Birfield axles (old Dana, Toyota, Rover, Dodge, Rockwell) don't hop around when turning because the joint is more like a CV. I have driven Land Cruiser with mismatched gear sets and they don't hop. But 10 miles of pavement will split a one piece case in two.
I've been building custom 4x4's for the last 25 years and I have seen it all when it comes to this question and my answer is always the same.
Always replace both sets. You cannot get away with replacing one only unless you are running a deicated mud truck with taller rear tires.