E350 Axle ratio

Sheep Shagger

Adventurer
I'm keeping my eye out for a new rear axle since I think my semi-floater is on it's way out and want to change to a full floater.
At the moment I've got 4.10's in a quigly running 33.5" tires. I really want to reduce my freeway RPM's and after looking at the OD units out there (and driven a few), I can't see myself going the Gearvendors or equiv route, so I'm wondering about 3.73 or 3.55. I very much doubt I'll run anything bigger than 34 or 35" tires, there is always 4low so I'm wondering why people don't do 3.55?
If I could be at or under 2k rpm at 80mph that would be perfect for freeway MPG, but I don;t think I'll achieve that, considering there is only a 200 rpm difference between 4.10 & 3.73 or 3.73 & 3.55, so best I could do it 400 rpm drop between 4.10 and 3.55 with 33" tires. I read a lot of people saying you'll trash your trans running 3.73 and big tires, but the 06 F350 (same year as my van same trans & ratios) came with 33" tires and 3.73 gearing, so not sure I believe that.

So what are the disadvantages for going from 4.10 to 3.73 or 3.55 ?
 

sdski

Observer
I currently have 3.73s and my initial plan was to keep them thinking that it would be best for fuel economy. I only plan to go up to 33's for now. However, I also plan to use my van in the mountains a lot, and I already need a little bit of a heavy foot driving around the mountains with the 5.4, so I'll be going up to 4.10s instead. I think the amount my engine will be working to go up hills with 3.73s vs. 4.10s will make up for the drop in mpg. I don't see myself cruising at 80 in the van either, so really I'd say it comes down to preference.

I would not worry about trashing your trans running 3.73s with 33s. I think dropping to 3.55s will be too much of an overall power loss. What engine do you have? Do you tow anything?
 

Sheep Shagger

Adventurer
I would not worry about trashing your trans running 3.73s with 33s. I think dropping to 3.55s will be too much of an overall power loss. What engine do you have? Do you tow anything?
Derr, I completely forgot to mention that, 6.0 PSD. I really don't pull much, most of the time it's just a 2k (loaded) motorcycle trailer. I do have a 20ft flat bed car trailer that I pull a very light car on, whole thing is 5k with car, but that's only ever locally and there aren't any hills round here.

Thanks to you and Chris for the reply. Think I'm going to try a 3.73 axle and if I like the change, change the ratio in the front axle.
I take it there is no harm in running a different front and rear axle ratio as long as you don't put it in 4wd?
 

sdski

Observer
I take it there is no harm in running a different front and rear axle ratio as long as you don't put it in 4wd?
You'll be fine in 2wd with different axle ratios. Give it a shot, I'd be interested to hear your opinion on the difference between the two. You've got a lot more power than me...
 

spencyg

This Space For Rent
Why are you opposed to a GearVendors OD? They are extremely reliable and allow a nice mix of higher gear ratios in the axle and good highway RPM's. Just curious.
 

Sheep Shagger

Adventurer
Why are you opposed to a GearVendors OD? They are extremely reliable and allow a nice mix of higher gear ratios in the axle and good highway RPM's. Just curious.
The unit itself is very nice, but for the price I'd expect a lot more.
At their asking price, there are two main issues I have.
The engagement is terrible. Really harsh.
Electronic controller is total crap. So outdated (both in function and components), needs to be redone so owner can change the settings rather than sending it off.
If it was cheaper, I could live with the above.

Other then than, For my Quigley, I'd have to modify my fuel tank, sizing it up with a friends GV unit before he put it in, I don't think it'll fit without notching the tank.
 

r_w

Adventurer
GV is a lot of money to spend if you already have an OD transmission.

My van has a 351 and 3.55's and it is fine for a flatland family bus with 31" tires. I would not want to pull or go up in elevation or through serious hills with it. If I redo it it is getting a 3.73.

You might be OK with a 6.0, 3.55 and 33's, but I don't know if I have ever seen a full-float with lower than a 3.73 anyway.
 

spencyg

This Space For Rent
Unless you're doing gears yourself (not advised unless you've got a very well equipped shop and have done it before), you'll spend the cost of a GV on a gear upgrade for the front axle and a "new" rear axle. To each his own. Good luck!
 

r_w

Adventurer
Unless you're doing gears yourself (not advised unless you've got a very well equipped shop and have done it before), you'll spend the cost of a GV on a gear upgrade for the front axle and a "new" rear axle. To each his own. Good luck!
That is a fair point...
 

Sheep Shagger

Adventurer
Maybe I'm missing something?
$1600 to get front and rear changed from 4.10 to something else, that's parts and labor. GV is 3.5k (unit + driveshaft modification) install ontop, but I'd do that except any tank modifications.

I need a full floater in either case. Seems they can be had for $500 to 1k, and maybe a re-build on top of that. Then reduce the $1600 (~$800) for ratio change to just front. Still seems for 2 to 2.5k I can have referb full floater with 3.73's front and rear.

So, like for like it's 2k cheaper. Adding on the axle and re-build still 1k cheaper, but I'll need to do that in either case. I think the GV unit is a better option as all I really want it to reduce freeway RPM. But personally I just can't justify 3.5k for something I think should be a lot better for for money.
 

spencyg

This Space For Rent
The need for another rear axle is probably going to be the driver here. It is the one thing that makes gear changes more practical in this scenario. If you were just looking to get better highway RPM's while at the same time maintaining a decent crawl / slow speed gear ratio, the GV would be a no brainer in my opinion, even with the slightly higher cost. In the end, I'll bet you'll find the new rear axle and the front gear change will come in at or over your estimate for the GV unless you're doing your own work and find a used axle in the boneyard for cheap. Up in the Northeast, if you can even find a shop to do a gear change, they charge $1000/end with materials. Used beat-up full float Dana 70's go for between $500-$1000, and then require a full rebuild (another $1500 minimum). You may just be working in a more economically friendly part of the country...if so, you're lucky :)

Regardless, looks like you have a nice little rig which is worthy of dumping some cash into no matter what the final solution might be.
 

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