I have an '88 F250 with a 7.3L IDI, 5-speed manual(ZF S5-42 I think), and 4.10(code 35) differential gears. I'm thinking about upgrading the gears to a 3.55 ratio or less, for lower RPMs at highway speeds. I've already got a GV Overdrive unit, so I can run at 60mph at like 1850RPM, but going to a 3.55 would drop that to around 1600RPM if I did my calculations correctly.

Do any of you have experience with that? Would I be able to just pull the differential(s) apart and stick new gears in, or would I want to get a replacement differential from a different vehicle? Are there gear-kits available that would do what I want, and what other considerations are there in doing this?

 

you are talking about a bit of money for both front and rear axle , and with 2 over drives you get about the same as you would with the 3.55 gears !
what size tires are you running ? will be cheaper to get some taller tires !

 

I don't know; whatever seems to be stock. Also, they've got about 1/4" tread, so they'll need to be upgraded at some point soon...

 

gear swaps in not something a backyard parts changer can do.
it requires specialty tools to properly set gear backlash.

plus gear sets alone cost around $400 per rear.

i would just upgrade to a larger 33 in tall tire

 

Funny, I'm going the other way with mine. I will have a set of 3.55 axles/gears when I'm done with my swap. Where are you located?

 

Hmmm, your gear vendors isn't working, or your tach is fubar, you should be doing about 70 at 1850, 60 should currently be right around 1580.
P.S. Stock tire is a 31.7 so here's some math.
1850/.78/.78/4.10X99.59/12/5280X60=69.94
^There is some argument about the OD ratio in the ZF5, I normally say it's .78 because that's what mine claims on it's side. But the difference between .78 and .76 is negligible.

 

ZF 5-speeds found behind gas engines have a .78 OD. Ones found behind diesel engines have the .76 OD ratio.

 

Here's a video that shows a little of what's involved in changing gears in a rear end. It's a newer truck, but the theory remains.

Rear Differential Gear Swap on Ford F-150 - YouTube

 

North-Eastern Washington/idaho area

Wouldn't surprise me. Here's a video of me at 55MPH - 20120106_009.mp4 - YouTube

Also, in double-overdrive 40mph is just over idle, and I don't have much acceleration until about 55 or so.

 

You already have the gear vendors unit? Very cool - you're set!

Unless you can find somebody that wants your gear ratio that will swap entire axles with you, I don't know that you will ever pay back what it is going to cost you to do this change.

How many miles per year do you drive? Figure gas at $4/gallon, and then figure out how much of an improvement this change is going to make (my guess, not much, maybe 2mpg). Now you can determine out how many dollars per year you will be saving. Then figure out how many years' worth of driving it will take for you just to break even on the swap.

It's almost never worth it, when you run the numbers. You're already better off with your existing setup than 99% of every other truck out there.

 

Ah, you're on the other side of the country... I'm near DC. I bet you can find a set if you keep your eyes out. A lot of guys do straight axle swaps and those guys usually think the TTB is junk so they treat it as such. I bet you could get a used front for a hundred or two. I got my front and rear for under $500 bucks with new ball joints and seals already installed.

 

I did this in my '89 7.3 5 speed. I found a junk yard front ring and pinion and had a shop install that. I swapped out the entire rear axle housing and springs from a 350 SRW unit w/3.55's. I forget the cost (4-5 years ago), but I got a 4mpg boost and reduced cab noise. I don't have an OD brownie in mine, but I would think that it would boost mpg even more. I am running 265/75/16 tires.
The downside is when I tow my backhoe (18K with trailer weight) I need to start in the low range if I am on a hill. I guess I could slip the hell out of the clutch to get it going in the high range.