(a little long, but please help me out here)

<!Ok, now that I've got my old F150 almost squared away (see sig) to be sold>, I'm going to be looking at doing some rear end work on my F250 (see sig).

The axle code on the doorpost is 35. The paperwork I got with the truck (bought a few months ago) says that it had 4.10 gears installed (at the dealer, I assume) when it was bought new. Coupled with the 351 @ 120k miles, I'm not liking the mileage--10 to 11 mpg. I do like the "getup and go" it's got right now, but I'm not a kid anymore, and don't really need it. Nor do I do alot of towing (but I may occasionally pull 5000 lbs or so, maybe 1-2x a year). So I'm thinking of putting in 3.55's or 3.73's. Any thoughts/ideas/suggestions? What kind of mileage increase would/should I see with this change for the 2 different ratios?

Also, I'm a contractor, so I occasionally have to pull into sloppy jobsites, and, this truck is a 1wd (why can't manufacturers just be honest about this?). Heck I got stuck on level damp grass with this thing (my own, arrrrghh). I'm fairly certain it's only an open differential, and I'm thinking that a locker would be the way to go over LS. I'm not looking to spend big bucks here, but for the ring, pinion and locker, I would say I could do about $600-$700 if necessary, installed myself. Which locker? What about expected ride quality? It's a 3/4 ton truck, so the ride is already harsh, which I don't mind in a truck, but I don't want something that's going to throw my rear end around in a turn or on an icy/snowy road. I like the idea of a manual locker, but I can't say I've seen anything reasonably priced along those lines, but would consider one if the price was right.

Thanks in advance for your help!

Jason

BTW, I would put new tires on with better traction, but the tires were brand-new (M+S street tires--don't recall brand)when I bought the truck 5 months ago, so I'll just wait on that. Even though my 150 has an open diff, the only time I got it stuck was when I took it sledding through some deep snow (she floated on the snow on my spare). I always kept good traction tires on the rear.

 

I would go with the 3.55 set up unless you have an auto trans. The rpm difference will be from 4.11 to 3.55 as follow and will vary depend on tire size and final gear ratio. 4.11=3224rpm 3.73=2925 3.55=2784 I just used some number sto show the difference in rpm from one to the other. I have a 3.55 10.25 trac loc rear axle for sale in the classifieds.

 

I do have an auto tranny (e4od), and I ran the numbers through the calculators on various sites. I'm looking for experience, mileage, etc. I think the rear end is an 8.8", 31-spline, but I'm not positive, that's why I included the axle code.

Jason

 

If you have an F250 you willnot have an 8.8 rear axle. it would never last under an F250 that is used like it could be used. They also never made an 8 lug 8.8. It would have to be from what I know 10.25. I am not sure if ford offered anything other than thew 10.25 rear axle in the F250 trucks. I believe earlier trucks like in the 70's had dana 60 rear axles. I do not know when ford switched. I believe ford did have a metric 8 lug unti for a while so that may be something to look out for.

 

You're right. Dunno where I got the idea that it was an 8.8. I dug out the manual, and the 35 code means it was 4.10 from the factory (open diff), and it's a HD model, so the ring is 10.25". So should I go 3.55 or 3.73? Detroit, ARB, PowerTrax?

Thanks,
Jason

 

What size tire do you run? This will affect the final drive ratio as well. If stock tires are all you drive on then 3.55 wouldn't be bad but anything less than 4.11 will cause the motor to work harder when towing. If it was me I would go 3.55 because I do alot of highway driving. Detroit is the best set up, in my opinion anyways.

 

As for a locker, you cant put a locker in a daily driven truck without the tires chirping and possible axle damage. A locker forces the tires to be spun at the same speed. Some lockers will ratchet around the corner, but its very harsh and not really great for a DD. A limited slip is the way to go. It'll spin both tires in the mud, snow, etc etc. About the only time it won't spin both is when 1 is off the ground, or there is a very high torque difference between the two tires. You can find a good LS on ebay or a store. Do a search here for limited slips, Auburn, Eaton, etc etc..any of the locker/ls companies.

 

Justin: I have a locker (Powertrax Lock Right) in my '92 F350, which is driven both on the street and off road. If I am easy with the gas pulling out of right hand turns, I don't get any tire chirping. How a locker behaves on the street depends on how you drive it. All automatic locking differentials (Detroit, LockRight, etc) differentiate wheel speed around corners by ratcheting teeth in the differential and it is not all that harsh once you get used to it, especially if you drive smoothly. I only hear and feel the ratcheting when making very tight turns, as in parking. It does make a clank that I can hear and feel when going from coasting to accelerating, but it's not that bad. Also, the new Powertrax NoSlip, along with the new Detroit Locker and Soft Locker are smoother and quieter than the old Detroit and LockRight, due to a spring cusioned engagement ring in the NoSlip and another mechanism that accomplishes the same thing in the Detroits. I do agree that a limited slip is quieter and smoother and better suited to street use than a locker, but lockers can and are used safely on the street every day (my neighbor also has an old Detroit in his truck and drives it every day).

Anyways, to get to the person posting the question... Jason, I know what you mean about the disappointment of having an open diff, my old Chevy 2wd dually was a peg leg and so was my F350 when I first got it. They both sucked in mud and sand, even the F350, which is 4wd. The LockRight definately made a big difference in the off road ability of the truck. It claws it's way through just about anything now when in 4wd and there are places that I go now in 2wd that used to require locking in the front hubs. The $325 I spent for the differential was well worth it.

As for the ratio change, here is my experience in changing the ratio on a truck somewhat similar to yours. I'm running 4.56 gears and 33" tires (~4.30 effective ratio) without overdrive (C6 trans), and yeah, I am spinning a few rpms at freeway speeds. With your 4.10 axle, stock tires and E4OD overdrive trans, you're not really spinning all that high rpms, by comparison. Anyways, I noticed an increase in gas mileage going from 3.55's to 4.56's because my truck is heavy (F350 Crew Cab 4x4) and my engine is small (351), so it was really working hard to get the heavy truck moving. The lower (numerically higher) gears took some of the stress off the engine and I don't spend as much time with my foot deep in the pedal as I used to, which saves gas. The only area where I lost mileage was when driving fast on the freeway unloaded, because I don't have overdrive. Mileage and power when driving in hilly areas or when loaded, and both, was better with the 4.56's. I'm happy with this combination, except for not having overdrive. A .78:1 overdrive ratio would put me right about 3.55 final gearing (3.35 effective, with tire size difference), which would be right where I was before I did the gear change, for freeway driving, which that truck was great for, as long as there were no hills to climb. With your 4.10 gears and overdrive, you're right about 3.20 final gearing, which should be plenty high for the freeway.

Since your truck is also on the heavy side, but not as heavy as mine, and your engine is also small and you do have overdrive, I doubt you will be saving much gas by going with the 3.55's unless you do alot of fast driving unloaded on the freeway or live in a relatively flat part of the country. I don't know that the small gas savings would justify the cost of doing the gear change. If it were me, and I was dead set on changing the gears, I would change them to 3.73, which is a nice balance between power and mileage, if running stock size tires. Of course I'm not sure it's really worth it to spend the money for such a small change...

One alternative to changing the gears, is to change to a slightly larger tire size. If you were to put 33" tires on the truck (285/75-16, I think is what is in metric), assuming your stock size is 235/85-16 (~31.5"), it would be similar to having ~3.90 gearing instead of 4.10 and would be cheaper than doing a gear change, if you wait untill you need to buy new tires anyways. That will lower your rpms a little and you will still have good power as well. I ran that combination on a truck I had and it worked good.

Anyways, hope that helps ya out some, or maybe just adds more confusion to the equation LOL. At least it's one more thing to mull over anways...

.

 

Thanks for the help so far. I do have stock tires (235/85R16), and the RPM calculators tell me that's 1977 rpms @ 65 mph in OD (which is close enough to the 2000 my tach reads). A 3.55 would get that down to 1712 rpms, and I think a 3.73 would be around 1799. I would think that would HAVE to translate to SOME fuel savings, going from 10-11 right now to even 13 would be great, especially with where fuel prices are right now, and would probably pay for itself in less than a year. SoCal--you've never been to IN, have you? Someone on another site calls this section of the midwest, "the foothills of Mount Level" I doubt I'll be doing much roadtripping, either, so this truck would stay on the flat mostly.

Jason

 

I know how the lockers work, I was just not getting into specifics to confuse people. MOST people do not find lockers pleasureable or acceptable on the street, which is why I said what I said. I didn't mean you physically can't, but it's very hard on tire wear and the axle. A limited slip would do better for a daily driver in comfort, and he doesn't really NEED a locker, as the aftermarket has some pretty tight limited slips. My auburn Ls/Locker (selectable) is very tight, it'll chirp tires sometimes without the locker engaged but it's still very smooth (it probably chirps cuz it's brand new).

There are tons of threads on locker vs limited slip....the search function comes in handy for that and I bet you can find a few if you look through the last 5-6 pages of this forum.

 

When I put in 4.56's instead of 4.10's with the 35" tires, I went down about 3 MPG in my 4x4 F150. I would go with the 3.55's for your configuration, you should see a increase in mileage with those gears.

As for as lockers go. I would not go with a locker for 2wd and mostly street use. They have characteristics that are best described as less than friendly for street useage ie...The spinning of the inner tire( you will see increased tire wear), the slack between deaccel and acceleration, a yank under hard throttle in a turn, and a locker could definiatly get you in trouble on ice. I would stick with a limited slip, it should help out alot for your truck and muddy conditions.

 

Jason, you're right, I've never been to IN. I'm used to driving a heavy truck with a small engine over hilly terrain and for that low gears are the answer, otherwise you spend alot of time with your foot on the floor and sucking up alot of gas. And Justin is right, a limited slip is smoother than a locker and lockers are not for everyone, but I happen to like mine and dont mind the little noises it makes. With a short wheelbase truck I can see where a rear locker could get squirelly, but on this long heavy truck, I haven't had any problems with it sending me skidding accross the road. I do have limited slips front and rear in the Bronco and the rear one is very mild, practically unnoticeable, on the street but both help alot off road.
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