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Discussion Starter #1
I blew my rear diff today on my 04 F250. Had it towed back to my garage and opened up the pumpkin, and found lots of shattered metal inside. So, my ring and pinion is toast! I bought the truck used last year, and not sure what gears are in there. The previous owner says that they are not stock gears, but he is not sure himself of what ratio they are. I am currently running 39.5" tires and I do a lot of towing. I have a one car flatbed trailer that I tow around (4000-5000 lbs depending on what car I have on the trailer). I drive mostly on the highway.

So, what are my options at this point?

What gears should I run?

Is it simply swapping out the ring and pinion to fix this, or is the damage more severe? I read some other threads and sounded like it might be better to swap out the entire rear axle when the ring and pinion fails?

I'm kinda new to this truck, so any advice or suggestions would be greatly appreciated. I was running 4:56's on my Tahoe when I ran 39.5" tires. Also, what front and rear ends come stock on the 04 F250's so I know what to look for when buying parts?

Thanks in advance!
 

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The rear axle is stout enough for what you say you are doing with it. Since you were likely running 3.73's in your 250, the tall tires, towing weight and likely heat from towing finally took its toll. I would swap in some much larger gears (4.56's will reduce your mileage, but then so will those 39.5" tires) and get an 08 rear differential cover (CSIPSD has reported a 20*F reduction with the finned aluminum cover versus a stamped steel cover).
 

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Discussion Starter #3
The previous owner claimed that the guy he bought it from had the gears changed out, but I'm not 100% sure. I know that the stock gears on the window sticker says that it had the 3.73's. Since my driveshaft is not spinning anymore, I guess I will not be able to spin the wheel to see what the actual gear ratio is (or was).

So, can I just swap out the busted gears for a new set, or is there more to it?

Would the stock 08 cover be better than some aftermarket covers like Mag-Hytec?
 

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That's a good question. You might assume that the Mag-Hytec would be the better solution since it allows more gear oil and more is better, right? Well, when it gets hot, the fluid will retain that heat and more fluid means heat is retained longer. That is assuming that the heat cannot be dissipated fast enough to compensate for the heat generated by the gears. I think that the Mag-Hytek looks great and likely performs as advertised to reduce your diff fluid temps, but I think that the 08 aluminum diff cover is "good enough" and since Joe has data to prove that it works, that makes for me. Good luck with your decision.
 

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If you tow alot...get 4.88's .

here is another way to find out what gears are in it.

Count the teeth.

Take the cover off of the differential, count the number of teeth on the ring gear and the pinion gear. Divide the number of ring gear teeth by the number of pinion gear teeth. The resulting number is your gear ratio. (For example, a ring gear with 39 teeth and a pinion gear with 11 teeth: 39 divided by 11 is 3.5454...; these are 3.55 gears)
 

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Discussion Starter #7
I honestly don't remember what RPM I was at, at 70mph. If I had to take a guess, I would say around 2500-2800. I haven't had a chance to count the teeth on the gears and do the conversion yet. Maybe tomorrow if I have some extra time.

I talked to a buddy of mine and he says that he thinks some F250's come from the factory with 4.10 gears. Anyone know anything about this? He thinks that the previous owner who had the gears done probably didn't do it right and that's why it failed. He also has a lifted F250 but on 38's and thinks these rear ends should be able to handle the bigger tires and the towing that I do.

I've had two different people tell me I should just find a used rear axle from the junkyard and slap that on. It would be a lot cheaper to go that route than to rebuild my rear end. Because, most chances are, there are metal shavings in the rear diff, bearings, and seals. What's your guys' opinion on that?
 

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i dont know what the previous owner did but it shouldnt be that off.... at 70mph with my 4.30 gears im at 2100rpm which is what i should be around stock... (37" tires) with your tire size i would guess with even 4.88 you should be in the 2200rpm range at 70mph... the lower the gear you go the higher the RPM.... some trucks do come factory with 4.10 but its usually the 350's and with a tow package.... most come with 3.73 rear ends

hope you get it figured out bud

-Raul





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Gear Ratio Guide for Larger Tires

Here ya go, this is a link to a chart on therangerstation.com. I think you will find this useful in your decision making process.

Gear Ratio Guide For Larger Tires
 

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Discussion Starter #10
OK, so I found the time to go underneath to do a little research. The part number I found on the ring gear is 10090-F10.25-4.56L-9x41. I counted all the teeth on the gears and its 9 on the pinion and 41 on the ring. So, based on the part number and dividing 41 by 9, the end result is that I have 4.56's in there.

With that said, I'm sure the front end is the same since I have driven it in 4wd before. Now, knowing all the facts, it probably wouldn't be a good idea to find a stock rear end at a salvage yard to swap in. I'll end up having to pull it apart anyway to change out the gearing. So, could I just swap out my gears for new ones or is there more to it? One mechanic I talked to said that all the metal shavings and debris from the old gears that failed might pose a problem if I just swapped out the ring and pinion gears. What's your guys' thoughts on that?

Thanks for all your help so far, it is much appreciated. Thanks for the link Ed.
 

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You'll be fone putting a new ring and pinion in your stock differential. Just make sure to clean everything real good. And make sure to put in new wheel bearings and seals while it's apart. That way you can clean the axle tubes also.:thumb:
 

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Discussion Starter #12
Looks like I will be taking it to Rearend Specialties in Santa Clara, CA. Going to run me about $900 for the rear end to be overhauled with a new ring and pinion gears, and all the seals and bearings. They said it would be a one day turn around. Anyone deal with these guys before? Price sound about right?
 

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Another option is to get a Dana 80 to put in there. Most come with 4.10's. You might be lucky to find a towboss axle that has 4.30's in it though.

Stouter.
 

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Discussion Starter #14
Got the truck back from the shop today, everything is as good as new and $900 in the hole. Just glad to have it back on the road! Now I just have to break it in.
 
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