1990 f350
Post Fiend
Joined: Nov 2005
Posts: 7,437
Likes: 4
From: Charles Town, W bygod Va
how about the axle code on the door jamb? I think 3.55 is most common but there are a lot that came with 4.10
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Senior User
Joined: Jan 2011
Posts: 184
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From: Hope,in
cool...so what i have is a 10.25 diff with a 4.10 ratio..... i found 2 places to get a ring and pinion set for it, 3.55 is as low as they go though... better than the 4.10 for mileage
More Turbo
Joined: Nov 2010
Posts: 521
Likes: 6
From: Cincinnati, OH
If you can afford a little more money, putting an OD would be best. Either an OD trans or a Gear Vendors add-on type. More money than the gear change, but then you still have the gears for potential towing. An OD would give more rpm reduction than the 4.10 to 3.55 swap.
Post Fiend
Joined: Nov 2005
Posts: 7,437
Likes: 4
From: Charles Town, W bygod Va
I'd be doing the math to see how long it will take to get a return on your investment. Maybe someone who has done the swap can give you an idea of mpg difference. I know for the driving I do (rarely over 60mph) I would rather have 4.10's. That being said, I bought a used set of 4.10's for my dump truck from ebay ($50 or so). Sold the 3.55's for the same price, IIRC
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More Turbo
Joined: Nov 2010
Posts: 521
Likes: 6
From: Cincinnati, OH
Guess my brain thought you had an earlier non-OD trans, sorry for the confusion. Back to my sleeping while appearing to be awake. 
Another thought is you could put larger diam tires and get a poor man's bump in gear ratio. Not all the benefit of 4.10 to 3.55 (13.4%), but you might get half that?
Cleaning up the aerodynamics can have benefit. Such as those large side mirrors exchange for smaller size. Get truck lower to ground and put an air dam to keep air from going under vehicle. Bed shell or bad top can help some small amount. It all adds up. Carrying excess weight does not help mileage either.
The benefits to lower rpms are more than just mpg, it also makes it quieter inside the cab, less wear on engine, runs cooler, etc.
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