AndyT

Ford finally lowered the price on F150's till I couldn't resist. My new 2008 has the 4.2 V-6, and a 5 speed, with a 3.31 ratio 8.8 rear end. I find these to be outrageously tall gears, as it is only tuning about 2200 rpm at 70+ mph. The obvious solution is to change gears, but I saw where newtakeoff.com is selling complete 9.75 rearends, in either 3.31 or 3.55 ratios. I like the idea of the bigger rear, but has anyone swapped a 9.75 in for an 8.8? Any ideas what you need in addition to the basic rear end? Driveshaft length and yoke design come to mind as potential trouble areas. Or am I just trying to make the truck into something it is not? Opinions welcome. I would probably have to change gears anyway, with the 9.75 rear.

nagata

Hey, I know exactly what you mean, as I have the exact same setup. Like you, I want to get some lower gearing, at least 3.73. I'm not sure what all would be involved in swapping out for a whole new 9.75 rear end, but I just got a quote to swap in 3.73 gears and add limited slip, and this shop quoted me about $1050 with tax. Pretty pricey, and I'm gonna get another quote or 2 (just for fun, I checked with Ford, and they quoted me $1550), but I'm pretty set on getting it done at some point, even if I do spend a grand. I'm also going to look into how much work it would be to do it myself, as I try self-perform as much maintenance and modification as I can, but I've never touched or even looked inside a differential before, let alone replaced gears and installed limited slip. IMO, the benefits of the lower gearing and limited slip (better acceleration, improved towing rating from 2400 lbs. to nearly 4000 lbs. according to Ford, much better traction especially since it's only 2wd) are worth the cost.

Out of curiousity, how much would it cost for a new 9.75? While I'm sure it would be a considerably stronger unit, I can't imagine the 4.2 putting out enough power, or allowing you to haul/pull enough of a load, to hurt a 31 spline 8.8" diff. Furthermore, jumping up to a 3.55 ratio isn't gonna make that much of a difference. Unless it's SUPER cheap, and won't require any custom work or outside of the differential replacement parts, I don't see the benefit, financially or functionally, of going with a whole replacement 9.75 rather than swapping out gears in your stock 8.8

08SC

What's the price on the 9.75? The price of a gear change is going to be high no matter where you go, the ring and pinion alone can be had for $125 - $200, then you'd need a shim kit, and it'd be easier to replace the pinion bearings and races, but its not necessary. Master install kits can be had for $50 - $100 though, which would give you everything you need as well. Total price for the ring and pinion swap should be between $300 - $500 total parts and labor, and between $900 - $1200 total if you go with ring and pinion and LSD. If you've got the money, you may as well go all out with LSD and gears, but I'd go with at least 3.73.... going from 3.31 to 3.55 isn't enough of a change for me to justify spending more than $50 on.... UNLESS the complete unit can be had CHEAP! I don't think anything should be different, as long as it's from the same year truck. Everything should bolt up the same, and the speed sensor should be the same, I can check for you, but I don't see Ford have two completely different set-ups just based on two axle sizes. Also, be aware that if your rear axle takes 75W140, that crap is around $20 a bottle for Motorcraft, and you'll need 3.

AndyT

They want $699 for the complete rear in 3.31 or $1100 in the 3.55. Guess the 3.55 is more desirable. I don't think the $200 ring and pinion sets are as durable as the OEM ones, so I'm thinking the real cost would be quite a bit more to change ratios. I agree that just going to 3.55 isn't worth it. I would be surprised if the u joint flange doesn't stick out further towards the front on the 9.75, but thats just a guess.

nagata

Whew, that definitely seems like a lot of money to increase to 3.55. The quote I was given came from a local speed shop, and they quoted Ford Racing ring, pinion, and trac-loc system parts. IMO, the best option is to swap gears in the stock diff. Whatever you decide to do, if you do change up the rear, post up the process and results, along with what it costs, so others can see the options that are available

FarmerFranck

Hey first time poster on f150forum.com! (im a v6f150.com member)

I've got a 2007 4.2 5spd with axle code 27 (3.31) and was looking to swap to a 3.55 limited slip. I simply fell upon this thread using google

Back to the topic, i want to do the swap mostly for the limited slip. I live where winters are harsh (canada), so i figure two wheels spinning is better than one! But what i was planning is going from junkyard to junkyard until i find the axle i want. Any of u guys considered this? Seems like i would save alot of money this way, althought i have no idea what an axle cost at a junkyard.

nagata

Welcome to the F150 Forum!

On to your topic, I had considered the possibility of finding a complete rear end out of a junkyard with 3.73 gears and a limited slip, but decided it would be very difficult to find exactly what I want in good condition. I have two points to make:

First, going from 3.31 to 3.55 gear ratio is a very small change, and unless you can do it for extremely cheap/free, I doubt you will be happy with the end result. Perhaps you will, but I think you would be better suited to go up to a 3.73 gear ratio, which will be a much more noticeable improvement in power, and shouldn't affect gas mileage that much. Personally, I've decided to go up to a 4.10 gear ratio, because since my last post in this thread, I've put on 18" FX4 rims w/ 32" tires (up from 17" rims w/ 30" tires), so the truck's acceleration/torque/hauling ability has significantly decreased. Between the bigger tires and 4.10 gears, I should end up with roughly a 3.83 gear ratio compared to original.

Second, after further research, I've determined that a stock Ford limited slip is not worth the money, as it is mechanically inferior to other types of positive traction differentials on the market. Please see this thread for more information: https://www.f150forum.com/f4/lockers-17244/ . As a result of the further research I've done and input from others, I've decided to go with an Eaton Detroit True Trac differential, which is a gear based (rather than clutch based as the limited slip is) positive traction unit. Yes, it costs around twice as much, but it should never wear out, and will perform MUCH better than a factory limited slip will.

Good luck with whatever you decide to do, and don't be afraid to ask more questions Stick around and join in on the various discussions, there's more information here than you'll probably ever need.

FarmerFranck

wow, thank you for that link, that was alot of great info! I will definatly research more into this, but the Detroit True Trac seems like a great option.

As for gears, i might even save alot of money and stay with 3.31. That way i could do the mod myself. I do plan on doing modifications to my truck (intake,exhaust,tune,e-fans), but my goals for this truck is fuel economy first, and performance second. I understand the benefits of having shorter gears (aka higher number), it would make the truck alot more fun to drive. Therefore on my next long trip, I'll do the enitre trip w/o using 5th gear. That way, i can see what shorter gears would do to my highway fuel economy. (i havnt punched in the numbers yet to see what 4th gear would equal to). If my fuel economy only slightly changes, im going with shorter gears. If fuel economy drops dramatly, ill live with 3.31 gears.

Again, thank you for all the info, and let me know what u end up doing aswell. I'm in no hurry for this mod, as long as it is done by next winter

driver444

I'm the guy that posted all that about the detroit truetrac. Don't even waste your money on the factory ford trac-loc. The True trac goes for $550 or so, and is absolutely awesome. As I said in the post, you don't even know its there til you need it, and it won't let you down.

Also, don't be so hard on that 8.8 rear end. It's been around forever. My 1990 F150 had an 8.8, and I had a 235HP 5.8 liter V8 engine turning it. That truck had 200,000 on it when I sold it, and the guy still has it. Never had a rear end problem.