Just got new tires for the truck, decided to go with the slightly larger 265/70r17 instead of the really small stock 235/70r17 needless to say the truck has to try a bit more getting started (loaded or unloaded) so i was planning on stepping up the rear ratio from 3.31 to 3.55, now the local pick-a-part has a 05, 5.4l f150 that the rear end looks pretty undamaged (and for $120). according to the door codes it's a 3.55 9.7" differential. My question is will this bolt up to my 08 4.2l v6.

Thanks and happy holidays!

 

I'm guessing your 4.2L has a 8.8" rear diff. And it's 2wd? The 9.75" will have a longer nose. Which means the drive shaft will be shorter.

Other than that, I have no clue what other differences are between the 2, someone else would have to chime in for that.

 

Regardless of whether or not it will bolt right up, which I don't think it will cause you have an 8.8" rear end, I don't think the effort will be justified. I'd want to at least go to 3.73s.

Look for a 4.6 rear end. Should be an 8.8 like yours and nearly all should have at least 3.55s, if not 3.73s, which is the only ratio I'd bother changing out to.

 

Alright, thanks for the suggestions. I'll keep looking around then see if I can't find a 3.73, there is a 30 day return policy so worse comes to worse, I have to return it and put my old one back on.

Thanks again for your help and preventing me from doing something completely retarded.

 

There are a lot of 8.8's with 3.55 gears, my 03 came stock that way. Look around a bit and you should be able to find one pretty easy.

 

Whelp, made it back to the junkyard, found a 97 f150 xlt with a 4.6l v8 only issue is that it has rear drums and I think it has different sized lugs. Just looking at it I'm pretty sure the disks off of my 08 would bolt up, and axles should swap over. Would anyone be able to confirm/deny this?

 

Stay away from the 97 axle. The shock mounts are different, the 97 has inboard where as the 04-up has outboard shocks. The spring perches are probably different, and the 97-03 has 5 lug axles with 5 x 135mm pattern and the 04-up has 6 lugs with a 6 x 135mm pattern. Finally the 97-03 had smaller brakes. On certain models you could run a 15" wheel, the 04-14 the smallest wheel you can run with factory brakes is a 17"

 

...and that answers that. Time to head to a different yard!

 

Why wouldn't this be a (near) bolt-in? The 8.8 and 9.75 were both factory options. OP will need a driveshaft, but I reckon if the 5.4 truck he originally posted about has a 4R75 in it, maybe the length difference is moot and he can grab that one.

In any event, I doubt 3.73s are going to be readily had aside from 4x4s and I doubt OPs v6 truck has that option.

 

The diffs are different sizes, different driveshaft lengths, and why would he want to get the driveshaft too? Though that's easier, it adds more cost for no reason. 3.55s are still not worth changing to anyway.

What? It doesn't matter if his truck could have come with 3.73s or not, if that's what you mean. You can stick 5.13s in the rear end and that wouldn't matter. And you 4x4 rear axles are the same as 2WD ones.

 

The best gears for a 4.2 are 4.10's, and you won't find those in a junkyard. Spend the money for a proper gear swap in your existing diff. You will also need a programmer to correct the speedometer/odometer.

 

I agree with this, which is why I was pushing for 3.73s at a minimum. But I was thinking if you can find a diff with 3.73s for $150, that's better than spending $1000 on a gear swap. 4.10s would be best, but you didn't bump tire size by much, so for cost to benefit ratio, a 3.73 axle is best.

 

yeah exactly, for $1000 I can tolerate the gearing for another year or so, (till i have the cash/time/money) but at $150 why the heck not?

Whelp unfortunately, besides the aforementioned truck there was only one other 04-08 f150 and someone had already tore into the rear end of it (only had 3.55's though) so looks like I'm waiting for right now. may toss my net a bit further and see if I can find anything. Either way thanks for all the help guys, really glad these forums exist.

 

Did you lift or level your truck when you added those tires? Not that you needed to, just curious how it looks. Have a pic?

I have the same sized tires, but with 3" rear blocks and 4x4 lenth coilovers lifting the front 1.5-2"

I have 3.55s in my truck and didn't notice any difference in acceleration from the smaller tires that were on there before. I can't imagine 3.31s and 3.55s would really act that differently.

 

Just took this before i went to the store, i'm standing on the driveway so it prolly looks a bit lower than it really is. Also that is a 2m/440cm antenna on the roof.

(cooper at3's, wooh!)

yeah, no lift or anything. Well besides the whole acceleration thing it would make the speedometer almost correct again